NIOS2IDE 9.1 questions

gdb can usually debug code compiled at any optimisation level.

At higher levels variables that only exist in registers may not be accessible by name, and static functions that are small, or only called once, will be inlined.

If you ever need to look at the asm, the optimised code is often easier to follow because it contains less faffing.

I would also always debug with the options you intent using in the release - otherwise you can get a sudden surprise!