I brew in bursts. One month I will brew five batches, then I will brew nothing for the next month or two. And my gf and I don't drink enough to finish the kegs in time for the beer. So I frequently prime my kegs in order to save CO2 (well, in order to save a trip to LHBS to refill my tank). I have been shocked at how long a small amount of CO2 lasts when I have pre-carbonated kegs. I have wondered if I over-carbonate, and reduce the pressure, will the CO2 move from my keg into my gas tank? Mostly I am just curious how many of you prime your kegs, and if there is any good reason not to.