I've never had a problem leaving 12g canisters in.

I've also removed partly filled ones - as long as you are careful and release slowly then you are unlikely to injure yourself. Not recommended for Crosman and similar with end caps but ok for pistols with capsules in the magazines.

The best solution is found on my QB78 Match rifle - when nearly empty you can use a key to open a valve to safely release the remaining gas which saves having to dry fire it.