Did you ever notice that tin foil has a shiny side and a matte side? Which side should go on towards the food and which side goes out?
Well, the official word from the Reynolds aluminum people is as follows:
"It makes little difference which side of the Reynolds Wrap aluminum foil you use--both sides do the same fine job of cooking, freezing, and storing food. There is a slight difference in the reflectivity of the two sides, but it is so slight that laboratory instruments are required to measure it."
So there you have it, it doesn't really matter.