Hiya, Sarg-
You pose an interesting question, and I will be watching this thread to see other answers.
For me, it depends upon the scale. Almost always, if the head is a separate piece, I will finish that on its own and assemble just at the conclusion. same with hats / head gear. If the assembly of the arms, etc will create hard to reach (and hard to paint) places, I try to estimate if such places will be seen. Shep Paine was once quoted as saying something like "If you can see it, you can paint it." Yes, Shep, but how well can I paint it?
Then, there is the question of "fiddly bits." For aircraft and AFV modelers, it is pitot tubes, radio antennae and such. For me, this includes feathers on hats, swords, hand held telescopes and other small, easily broken parts. These usually will be painted separately and added at the end.
I look forward to seeing the wisdom of other planeteers posted here.
Thanks for asking the question we might all have been thinking. Keep calm and model on!
N