Someone once told me that I should drain the pressure out of my air compressor once I am done using the tools. Does one really need to do this? Someone once mentioned it had something to do with condensation in the tank, but I'd really like to know canonically what the safe/proper procedure should be.
It is a good idea to drain the air compressor periodically. Part depends on how much you use it. My grandfathers tank hadn't been drained for years (I think after he quite using it but everyone else still did) and it had 20 gallons of water in it.
The big problem is the water rusts the tank from the inside and on top of that if you have a lot of water (quarts/gallons) in the winter time it can freeze and possibly damage the tank.
How fast it accumulates depends on how humid it is where and when you use it and how much it is used.
On my little tank (10 gal.), it only takes a minute or two to open the pitcock on the bottom and let out the water. I generally try to do this at least once a year in the fall, but if I'm using it frequently, I try to remember about every 10 hours of use (that might take me two months).
After doing this a couple times you get the feel for how fast moisture collects and plan accordingly. But doing it more often isn't a big deal other than the time you spend on it.