I like to paint the parts before I put it together. It makes masking and painting easier that way.
The only drawback is that it takes planning ahead to know how the paint will look when the pieces are all folded into the plane.
Like in this video:
https://www.youtube.com/watch?v=M7mK3NIPTgA
This is the best paint job I ever did:
On the other hand, Flitetest has done some pretty simple paint schemes where the plane is painted either with a little bit of masking, or with just freehand blending after the build is complete.
Like this:
A few pro tips for spray painting:
-Better paint even at Walmart like Rustoleum or Krylon will look way better than the cheap-o cheap-o $0.99 stuff. The cheap stuff is almost watery. Better paint will be thicker, and will make a better sheen.
-Do like 3-5 really light coats!!! It is easy to wrinkle foam board with too much paint. When you're doing your first coat, if it looks like more than a light dusting, ITS TOO MUCH. DUSTING is the key word. 2nd coat can be thicker, and so on. Try it on a scrap first. It's a real bummer to ruin a speed build kit.
-Get good painter's masking tape. paper on foam board (not so much the waterproof stuff) really likes to stick to masking tape.
-Put the masking tape on lightly. You're just shadowing it from the paint. No need to press it down too hard.
Take your time and it can look great!!

Don't get too wrapped up in looks though, the more work you put in to it, the longer it is before you can fly, and the more painful the crash. JUST HAVE FUN AND BRING A FRIEND!