Do WHITE LEGO elements fade or yellow while sealed for years, or do they stay their perfect white original color?
Should be Fine
The sealing process, at least in the new bags without perforation, keeps the bricks safe from aging. The Taj Mahal should be just fine, and even opened, my Imperial Shuttle bricks are great!
In fact, all the newer bricks, 2003-2007, seem to be pretty yellow-proof. You'll typically only find those in much older sets.
Yellowing occurs because of UV interaction with the bricks. Box, indoors, away from windows, any of the above should keep them plenty safe.