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If you look at a function in the Disassembly window, you should be able to see what registers are being pushed/popped. The compiler optimizes to only push/pop relevant registers I believe, so you can double-check to see all the registers which are being pushed/popped are actually used within that function.
If you want to optimize it, I think the best way to do this is the create an assembly version of the designated function so you can determine what registers you actually need to use. I don't know if there's a CCS setting to optimize the stack usage the way you're describing, but I will forward this to the compiler expert for them to answer.
