Ideally I think you would get it checked out at each of the seasonal extremes you have noted. You may not always need a set up unless you keep really tight tolerances. The more fussy you are, the more likely you will need set up.

Acoustics I don't find need too much seasonally, again depending on how low your action is, how you like your neck, etc. Electrics squirm around a bit over the seasons, but you should mostly only ever be looking at a little truss rod tweak for the season.

It's all relative. :) The next person might give you a list of 20 things that simply MUST be checked and tweaked.