I keep the seals oiled and always leave gas in, I often leave an empty CO2 in often for months at a time. I'm not saying this is the right thing to do and everyone has a different opinion. I've posted this before but the way I see it is this:

The seal is designed to expand and contract in order to give a gas tight seal, it is made of a material designed to do this. As long as you avoid making the seal brittle it will do its job.

One way of avoiding the seal getting brittle is to keep it moist with a suitable oil that will not leach the plastisizer out of the material, when you gas up a gun put a drop of this oil (I see nothing wrong with Pellgun oil, others might have an alternative that is cheaper) on the CO2 and the oil will stay in place as long as the CO2 is there.

The other way of making it brittle is to make it hard. You can avoid this by making sure a CO2 cart is empty before removal. Once the CO2 is removed and starts to vent onto the seal it will cool it, this will obviously harden the surface of the seal and also causes the seal to contract (from the temperature change which can be extreme) at the same time it is expanding (as the pressure from the CO2 cart is relieved).

So when you combine a dry seal, that is being contracted by freezing gas while simultaneously being expanded by the drop in physical pressure being placed on it you quickly end up with a dodgy seal.

My thoughts, keep em lubed, shoot it till empty before removing, but do not worry if you leave it gassed or with an empty CO2 in it.

I've never had a seal fail in any of my 22 CO2 guns (many with multiple magazines).

Others may disagree and that's fine, but this works for me.