Stainless Valves in YZ250F ( Rich or Eric?)

I am replacing the valves in my '03 YZ250F with the kibblewhite stainless. The exhaust valves are still in spec as far as clearance is concerned and the are no "visible" signs of wear. Should I go ahead and replace them as well? It will only be an additional $75.00 in parts to do it now, but I've read (other places) the exhaust rarely give any problems. Would I be stupid not to go ahead and do it or is it possible they will be okay as is for an extended period?

I know the CRF's already use steel exhaust valves -- do the Yamaha's?

Thanks!
Len

Thanks Rich!