Since I use my PM9 for personal protection, I always keep a round (cartridge) in the chamber at all times. I do not worry about the extractor spring, since it does not go through a great number of cycles. Constant compression is not bad on the spring. It will however fatigue if it goes through too many compression/decompression cycles.
Since the PM9 will only discharge if you pull the trigger, I am not worried about it constantly being loaded.