If you are defining the word
need as "If I use a light weight pellet on a under 500fps rifle will it break the spring?"
the short answer is no. It would not be as detrimental as cocking your rifle and pulling the trigger without a pellet in it.
Which would cause the piston to slam against the compression tube body and causing unwanted forces on everything.
Might break the piston, seal, or spring.
But even using heavier pellets still puts stain on the piston/spring components.
Not sure what you are trying to accomplish using a light weight pellet...Do you have a bunch and just want to use them up? Or trying to gain more FPS???
No way will a light weight pellet be as accurate down range as a lead one. (Read the Book)
There are So so many articles and videos out there on choosing the right pellet. You need to do some searching and do lots of reading.
A great book to read is
The Practical Guide to Man-powered Weapons and Ammunition by Richard Middleton
A great read and for $10 it is worth. You'll learn at lot.
But for now just watch these...
Selecting the PelletEverything You May Not Know About Spring Piston Air RiflesHope that helps.