You may occure leaking, but it would be very little. If you has a full CO2 tank and left it alone for 20 years, I would say that some leak wouldn't be abnormal, but if it's just for a few months or so, you should be fine.
Oh ok thanks, I don't plan on storing it more then 1-3 months. I just didn't want to put the tank back on after storing it for a couple of weeks or a month and find out that the tank is almost empty.