Hello!
That question keeps comin' up all the time on the forums. The answer is usually: "It depends!" If you think you can cut it, paint after all is glued up - this way you get stronger glue joints. If you want to paint something that would be unreachable after glueing, paint first, glue later.
In your case - a tank - a big factor would be if you want to build it "factory clean" or "field dirty". My take is if you build a dirty tank, it's mostly no good to paint clean colours first and then put dirt on top of that - best way is to paint it dirty right on. That would mean the whole running gear is mostly dirt colour with just a hint of raw metal or rubber looking through. In this case it would be perfectly OK to build the whole running gear, then paint it dirt colour - with airbrush this will be easy - and then just pick out the centers of the wheels in OD and just a hint of rubber tyres on the outside of the road wheels and some worn metal on the track teeth and the drive sprockets, and there you go - weathered tank!
I hope this helps you - thanks for reading and have a nice day
Paweł