I worry about it some. All my sets are in sheets in binders. Very few are out in holders exposed to a daily light source. There are a couple of good threads here where Ben has posted examples of cards he has exposed to ongoing light sources to show effects over time. There are also some related discussions in the various 58 Blue Aaron threads.
The Blue Aarons were initially selling for a big premium but so many showed up eventually that it may be someone was creating some or all of them with light. If a sheet defect you would expect more examples of other cards from the sheets.
I picked up the Mays below because I thought it might be a print defect given both the front and back differences, but it is likely also due to light exposure. Maybe Ben can re post some of his light exposure examples. In one thread someone posted a card that was faded from the light except for one large round area where there was probably a sticker with a price on the holder blocking the light source.
