Priming parts on the sprue?

I think its a matter of personal preference and build style.  I never prime anything while its still on the sprue.  The reason I don't is because I like to give each part individual attention to make sure I get all of the flash off and remove pin marks from areas that will be visible.  Also, there are a lot of subassemblies that it makes more sense to have cemented together...seams removed, etc. before primer goes on.  To hold small items for priming/painting/drying, I use toothpicks and a hot glue glun.  Chop off the point on one end to get a decent surface area for the glue, attach the part at a spot that won't be visible, prime/paint while holding the toothpick, and then you can stab the pointy end of the toothpick into the edge of a roll of cheap masking tape to give it a place to sit while it dries.

Sometimes it makes good sense to me to prime and/or paint things on the sprue. A good example would be fuel tanks, bombs and other munnitions. I find that by removing as many attachment points as possible, gluing and priming, I've got parts that are (mostly) ready for paint, and still have a "handle". I like to use alligator clips on skewers to hold small stuff for paint, sticking the sharpened ends into styrofoam blocks.

Other assemblies can benefit from staying on the sprues, if it's easy to mask glue attachment points. 

i usually do, but then you need to scrape off the paint if you are using solvent glue. THat isn't hard though.

Bill

I frequently do that. Saves time.  Does require a slight touchup at sprue attachment points.  I dont bother to prime these areas, usuallycovered during painting, but may a slight touchup with fine brush.

I would say this;

 Each of us has a preference. Now in reality I would say Ditto to Fermis and Bill. It's overall easier that way. I have re-learned that lesson on the two 1/48 scale A-10s I am piddling with!

I almost always prime and paint on the sprue.  When I started modeling I found how to hold the part a challange.  The sprue creates a nice handle.  

I really can think of only a handful of instances where I would want to prime and paint on the sprue,perhaps landing gear,landing gear doors,the basecoat of a fuselage half but usually I'm assembling then painting.

Depend a lot on the subject 

I like using a spure  as a handle when I paint parts, especially small parts.

This is a photo of the parts for a 1/12 scale Tamyia kit. There are numerous  very small parts. I would find it very difficult to paint those parts after they were removed from the spures.I painted the parts while they were still attached to the spures and did minor touchup as required. In fact, the only only parts that I painted after I removed them from the spure were some of the blue body panels.

I pretty much 2nd what Johnny K said, his mo is same as mine in general. You do what you gotta do that's all. But one way or another everything but chome or clear parts get primed. And quite a lot of parts get color right on the trees/spru.