I see this question get posed on the forums a lot. First of all, I want to say that it doesn't matter. The high pressure gauge (if you have one) will read higher when its out of the refrigerator, but that doesn't mean anything. Also, if you have your tank outside the fridge and move it inside the pressure will appear to drop 300-500psi. This does NOT indicate that you have a leak.

There are benefits to doing it either way.

Personally, I keep mine outside the refrigerator. First and foremost, it just takes up space in the refrigerator. If I wanted to keep my tank inside I would lose space for at least one keg. It also allows me to more easily change the pressure on the regulator since I don't have to open the refrigerator. I can also look at the high pressure gauge every time I pour a beer so I can see if it starts to drop. Having the tank outside the fridge also allows easier access to my CO2 utility line. Finally, I can easily change out my tank when it is empty.

Many people keep them inside for several reasons. Aesthetics is one reason. If you have your kegerator in a visible place in your house, it is nice to hide the tank, which is usually not attractive. Mine is in my basement, so I don't care. Another big reason that people keep it in the fridge is that they are afraid of drilling through a coolant line and ruining a perfectly good refrigerator. On mine, I drilled through the bottom where I could see that I wouldn't damage anything. Also, if it's in a refrigerator, it is generally protected from falling over.

If you do want to keep it inside and you're eyeing up the old meat drawer on the bottom, think again. You can't keep the tank on its side. Doing so could release liquid CO2, rather than gas, and could damage your regulator.