This question is asked frequently... it makes no difference which side is in or out... the shiny vs. dull side is simply a result of the manufacturing technique... While it is true that the reflectivity of bright aluminum foil is 88% while dull embossed foil is about 80%, it's equally true that cooking doesn't involve visible light but, rather, infrared (which isn't visible) and isn't affected by "reflectivity". Additionally, have you ever seen a smooth, non-wrinkled piece of aluminum (yes, that's the correct spelling here in the U.S.)? If there's any reflectivity involved it stands to reason the wrinkles would destroy any effect on the item cooked.
Alcoa (the largest manufacturer of the sheet metal) will tell you that it doesn't make a difference which side is in or out when using foil. But, use whatever method turns your crank!