Yup, stormtroopers most definitely grow yellow over time. One of my troopers is starting to show some yellowing, but only on certain parts. Might be having something to do with the plastic mix. Pleather holster is also starting to show some wear. My collection is placed in a cool room with tinted windows, no direct sunlight and most of the time the room is dark. I live in Sweden so the air is pretty dry most of the year, which could be one of the reasons behind the deterioration.
Anyway, that's my experience with troopers. Others might have different stories with more favourable development, so take mine with a grain of salt
i think it's more because of the plastic oxidization and not much to do with paint. though white paint may turn yellow due to environment, plastic tend to oxidize and turn yellowish. the medicom troopers are still pretty white, not sure about sideshow, but HT troopers are relatively new compared to them.
and yea if the air is too dry it's also bad. too cold also bad as moisture evaporates from them turning them brittle.