A lot depends on the subject, a car engine us usually lots of different small parts that need to painted separately it can be easy to miss a bit so I always get the parts together and test fit before paint.

Body parts are much easier

Again w aircraft there tends to be lots of small parts that all go together inside a large body and there is nothing worse than seeing a part on the bench you forgot to put in so I always keep the parts in some sort of tray or organiser so nothing gets missed.