Always good practice. The thing turns into a giant greenhouse especially if the sun is fairly directly into either side of the plane, much less if the plane is nosed into the sun or tail into the sun. I know that some people are very uncomfortable walking in to an airplane that is like a meat locker (I actually prefer it) but walking into one that's a sauna is infinitely worse, because then everyone starts sweating and stinking and folks get really crabby.

--PP