Brian, I know it is counter-intuitive, but the scientific evidence points to keeping a spring airgun cocked for long periods (even months) generally has only a negligible effect on the performance of the spring once it has been settled in. It seems that most of the weakening and shortening of a new spring occurs within a short time of its use, and after that nothing much will happen if it is then kept under compression. Any further degradation and distortion of the spring that is found will be mainly caused by its repeated compression and expansion under normal use.

However, this should not be taken as a recommendation to keep a gun permanently cocked for obvious safety reasons!

I suppose another factor could be the risk of the sear failing due to metal fatigue if kept under constant high stress.