Having the co2 tank in or out of the fridge will have no effect on its function or your beer. It's really more of a convenience thing. If you have room in the fridge, it keeps it out of the way so you don;t trip on it, and make it look a little nicer (you probably don't want a co2 tank sitting out in your living room). On the other hand, having the tank outside may make room for an additional corny keg. I was using a 5lb tank and had inside the chest freezer, but I have since switched to a 20lb tank and keep it outside. Now I not only have a larger tank that i can refill cheaper and not as often, but I can also add another keg to my setup.