There are several schools of thought on this, and it just depends on how you feel about it. Smoking rivets could be a concern, but if you're building properly and having someone look over your work before you get too far, this concern can largely be negated. Most of the smoking rivets I've seen looking over aircraft at fly-ins are on the empennage. That's where you're beginning the learning process and are often most likely to make mistakes.
The bigger reason to wait may be changes you have to make after you start flying, such as cutting the aft end of a wingtip to shift it up or down, or maybe adding trim to the rudder. Depending on the model, there potentially could be other modifications you need to make that are only discovered after the aircraft has flown. Again, carefully following the plans and striving to keep everything properly aligned during construction will help to ensure that the plane flies straight & level with the ball centered. Even then, some of the recommended corrective actions can be done with the plane painted (e.g. squeezing trailing edge of ailerons).
As far as painting early, it's easier to paint the aircraft in pieces--assuming you're doing the painting yourself. The wings can be put in jigs or rotisseries to make painting easier, and the same goes for other components. Since there's no oil from the engine, everything is clean from the very start. If you're taking it to a paint shop prior to flying, the aircraft obviously has to be transportable. Depending on the complexity of the paint scheme, it could be difficult to maintain alignment of stripes, paint colors, etc. with a disassembled aircraft. Of course, if it's a solid color, that's a moot point.
Personally, like Todd, I waited. Painting was not my gift and I was ready to start flying after a long build.
