You can also put a fan in a collared keezer if you are worried about the cold air stratifying. Some of it depends on where your temp sensor is. If the sensor is at the top, the beer at the bottom (where you are drawing from) will be several degrees colder than the set temp. If the temp sensors at the bottom, you get the opposite (warmer beer at the top of the kegs). I figure most people without fans just put the sensor in the middle and aim for the best, then adjust if necessary.
I guess a fan would also help if your shanks were getting warm and you were getting more foam in your beer than you wanted. I probably wouldn't worry about it at first, and if you decide you want one you can make one pretty easy out of a computer fan.