I'm in agreement with Steve. Shoot your most accurate pellets and enjoy it. Sure, springs and piston seals will eventually wear out but usually after tens of thousands of pellets have gone downrange so no reason to let something so far away intrude on your enjoyment.
With that said, many times it works out that the magic pellet is one that produces something very close to max energy for a given spring or gas piston power plant. Logically it is an indication of where the power plant is working most efficiently, and less prone to piston bounce and vibration. More of the spring's stored energy is going into propelling the pellet. So if you want to get some sense of the optimum weight range for your particular rifle, simply chronograph a range of weights and look at the resulting energy for each. What you'll find is that very light and very heavy pellets produce less energy (FPE) than those in the middle.
If you are interested in the academic side of it, to my knowledge the most credible explanation for why too-light or too-heavy pellets are more stressful to a springer powerplant is made in Cardew's book "The Airgun: From Trigger to Target".
The progression is something like this:
1. A "too light" pellet starts moving too early which leaves an insufficient air cushion in front of the piston to help decelerate it, thus it slams into the end of the compression tube. This abrupt impact is stressful to both the seal and the spring.
2. A "just right" pellet allows the piston to come to a well-behaved stop at the end of the compression tube.
3. A "too heavy" pellet stays in place too long and the pressure builds much higher, causing the piston to bounce back off the cushion of air in front of it, forcing the front portion of the spring backward while the momentum of the back portion of the spring is still propelling it forward. That can cause coils somewhere in the middle to collide with each other. Meanwhile the elevated pressure simultaneously creates a greater temperature spike which promotes detonation of any trace hydrocarbons in the compression chamber, forcing an even more violent reversal of the spring and potentially cooking the seal.
In practice, there does appear to be a more violent recoil when using super light or super heavy pellets in most springers I've shot. And it stands to reason that given the relatively long dwell time a pellet spends in the barrel, these "extra vibrations" are not good for accuracy...even if we have no concern for the longevity of the spring and piston seal.
