I never noticed a change going with seat-of-my-pants analysis, although chrono results did decrease over time. If you always de-caulk the gun after use (shoot one shot without a BB in the chamber) it should chrono about the same for over a year. It took maybe two years without de-caulking the first year to go from an [estimated] 280 when new to 240 which was my slowest/ last chrono test.
Besides, springs are cheap ($10.75 for an M100), but the install is tricky until you get used to it
note: caulk=c0ck, stupid censoring.