I agree with the guys above, it is all up to each modeler's choice.
I build in batches, though. I do it two different ways, depending on the aircraft I am building. One way is to build 4 Skyhawks (or whatever) at the same time, then when I dig out the seat belt color, I don't do just one belt set and then stop, I do 4 sets. The other way is to do 4 completely different aircraft Color schemes together, then I never have to wait for paint to dry, by the time I do something on each model, the paints are dry or the session is over.
I also use a lot of underwing weapons, so whenever I hit a stopping point on the planes and still have modeling time left, I just do a dozen of some weapon type.
Rex