We were just talking about this at the field today. One guy said he heard that the combustion products which reach the gas tank through the vent line are enough to contaminate your fuel supply. However we both still drain out tanks back into our fuel reservoirs. Thinking about it now, either way that fuel will run through the engine, so I don't understand the difference.

I drain my fuel back into my supply (to remove the possibility of water accumulation, and so when I turn my plane upside down, or nose down to carry it around, fuel doesn't pour into the engine). I run the engine until it's totally dry, and use a few squirts of after run oil. This is what it says to do in my OS 46AX manual, and that's what I do. I have the best running engine at my field, hands down, and it burns a gallon of fuel per week (has some hours on it). It starts with one, two flips of a stick at most, so I intend on continuing this practice.