Since I never wrote timing-critical code for the 6502 (apart from "make it as fast as possible") I cannot recall many specifics. Since you did, you certainly have a better understanding of how it worked.

I am restoring a 65c02-based //e clone, so, I may be able to properly measure instruction timings, but I won't hold my breath.

Ye well, it could be C64-specific quirks since it shared the bus with the graphics hardware.

Sounds like a fun project.

It seems that all the mysteries of 6502 timing have been revealed thanks to the Visual 6502 project http://www.youtube.com/watch?v=H_15RtVbqGU#t=5m33s http://www.visual6502.org/ .

Yes you are right, the instruction timings were very exact as far as I remember. The only cases where there was an option was in the case of a branch taken or not.