All tanks have a burst disk if the pressure gets too high. There used to be issues with CO2s tanks unscrewing from the actual valve and turning into rockets. All current tanks are made with channels in the threading that will alert you if that happens long before it becomes an issue. As long as a full tank doesn't get too hot (like leaving it in a hot car) it isn't an issue leaving it pressurized. HPA tends to loose a bit more pressure if you store them pressurized, but they also have a burst disk safety. HPA tanks keep a relative consistent pressure, so the only need for the disk is if the regulator fails. They too have a problem turning into rockets, their high pressure and the added length of a regulator has caused a few problems for players attemping to do a superman dive and landing on the butt of their tank.
If you don't have an on/off valve or slide check, you can point the tank away from you and depress the pin using a flat head screwdriver. Shooting works pretty much just as well though. In the end you still need to press on the pin to get the last bit out.