I once bought a used C02 pistol that have a deep imprint on the C02 bulb seal, which did not seal particularly well when the bulb was loaded, for this reason I prefer not to leave the bulb in for extended periods. The point made about not releasing/venting off CO2 onto an expanding seal seems very valid.

If I occasionally leave a CO2 bulb in an unloaded pistol for a few days (to finish off the gas later), I place a rubber band around the barrel and trigger guard.
This way if I go back to it later than planned, the band serves as a reminder that it is gassed up. The Postman used to drop red bands all over the place which were ideal.

I am convinced that the Umerex bulbs also contain a lubricant but have never seen it mentioned when on sale.

There are a few threads on bbs where folk have had the valve seal stick open, causing all the C02 to vent down the barrel. In some cases, they found that Pellgunoil resolved the issue. I am a great believer in the occasional use of a lubricant (Pellgunoil or a bulb containing lubricant) after having a problem with the valve seal in an SSP pistol that leaked. Strip down showed that the internal valve seals were bone dry and failed to seat properly; Pellgunoil fixed that.