I'd clean it often. I don't have a Velocity but do have a Sotar and it uses basically the same tip. Don's right, they are kinda small and delicate and acrylics will build up inside the tip. You can't flush it all out. But you really don't have to tear it down every time. My other brushes I do but since that tip is so small and easy to drop I don't tear it down until performance drops off. After you get used to using it you'll notice when you have to start pulling the trigger back further to get the amount of paint flow you want. Tip dry will come on sooner and it will clog sooner. And when it does...
Flush it out well first then unhook the brush and take it to your workbench. Don't hold it close to the edge, hold it back over the bench a ways. Arms length if you can. 
Just don't get too close to the back edge of the bench, table, whatever. Screw off the handle, back section of the brush, and loosen the needle chuck and pull the needle back an inch or so and tighten the needle chuck again. On the front of the brush there are two parts that will screw off, the air cap and the hold down ring. You'll see two lines of knurling. May look like one piece but it's two. You can take them off one at a time or both together. Whichever way you do it, when you take off the hold down ring you want to be holding the brush with the nozzle end pointing straight up. If you've never taken it apart before they may be tight but there's several threads on each so once you break them loose it will take a couple of turns to get them off. Once you get the hold down ring off you'll see the little tip/nozzle and it just sets in it's holder. If you tip the brush over it will fall out so, have a cloth or paper towels on the bench because it can roll or bounce far away. And it's very hard to find.
Now, go back and loosen the needle chuck again. If there aren't any grooves or anything around the back end of the needle you can push it forward and pull the needle out the front of the brush. If there are grooves I would just slide it forward enough to wipe any paint off the needle and then pull it out the back of the brush. If it's grooved the edges could knick the needle bearing/seal. Just damage that would accumulate over time.
Here's where an ultrasonic cleaner might come in very handy but you'll still need to tear the brush down. I'll go ahead and explain how I clean.
So, if the air cap and hold down ring came off together I'd separate them now and drop them and the tip/nozzle in a little bottle or jar of alcohol or thinner and let them soak for a while. Just wipe the needle down well to clean it. Now that the needle is out of the way you may have some paint build up in the bottom of the cup on the brush so you can use a paint brush with alcohol or thinner to clean it. You can use a toothpick if you need to scrape anything. I use those little interdental brushes to clean the passage at the front of the brush. They're pretty cheap and last a long time.
There's really nothing to clean on the hold down ring and any paint build-up on the air cap will probably be on the outside so it's easy. To clean the inside of the tip/nozzle you can use a nice pointy toothpick. I have to use a pair of tweezers to get the tip back out of the jar. Unless you sand a toothpick down or just happen to have one that's very pointy it probably won't stick all the way out of the tip/nozzle, the passage is too small. Just insert the toothpick gently and don't force it. Once it's in give it turn or two and take it out. Keep doing that till you don't seen any paint on the toothpick. When you're at that point then wet the tip/nozzle again and insert the tip of the needle and gently turn it, work it back and forth and then flush it out. (An ultrasonic would alleviate all that.)
Some folks may scream but here's what I did/do to clean the tip/nozzle instead of using the airbrush needle. I found a sewing needle that was small enough for the point to stick out the tip but large enough not to go all the way through. Then I filed about half the thickness of the needle away and removed any burrs and cleaned it up real nice. Then I put in a pinvise, slip the tip/nozzle on it(gently) and give it a couple turns, flush and do it again. Those tips are very small and it can be very difficult to get all the paint build up out. You may not want to try that but, that's what I do.
Now, remember that the only thing that's holding your trigger in the brush when the needle isn't in the brush is the pressure from the back lever. Before you put the brush back together you want to make sure the trigger is seated. If the back end of your needle is smooth and you pulled it out the front of the brush you can put it back in that way and not have to worry about the sharp end of the needle getting knicked. Again, just be gentle and take your time. If you put the needle back in from the back of the brush just go slowly and make sure the trigger is seated and down. The needle tube and chuck will guide the needle and the only things you can really knick it on are either the trigger or back lever. If you're going to put the needle in from the back then go ahead and put the front of the brush together first. Set the tip in the tube that holds it, then screw on the hold down ring and then the air cap. Snug them both down but don't overtighten. If you put the needle in from the front then pull it back far enough to be out of the way like you did when you tore it down and then put the front of the brush together. Then loosen the needle chuck and slide the needle in slowly. It should just barely touch. Tighten the chuck, screw on the handle and you're ready to go.
Be careful with the needle because it is very sharp and doesn't care what it sticks in. If you do happen to bend or knick the tip of the needle you can fix it. Long as it doesn't look like a fish hook or break off you can usually straighten and polish it. If that happens then come back and I'll write you another book.
Sorry for being so long winded but maybe it will help. And as I said, I don't have a Velocity but my Sotar tears down almost exactly the same way and uses the same nozzle/tip set-up. If it weren't for that little bitty tip you could do it all in less than 10 minutes, 5 after you've done it a few times.The exploded view in the manual will show everything. If you don't have it you can find it or an exploded view and parts list online.
Tony
PS...One other thing. With a .21mm tip you're going to have to thin your paints more. Maybe a lot more. If you have trouble with it (or just want to try it) the Renegade Rage .33mm tip and needle will work in your Velocity.