All the currently available PC fighters work with 6 buttons.
The extra buttons just post combos of the core 6.
I only ever map the main ones because it's more true to arcade gameplay.
Some require you to map 8 before allowing you to save your control mappings though, which is a slight PITA.
If building a cab specifically for PC fighters, I'd go with 8 and use an interface that shows up as an xbox 360 controller.
IMO, it's the only way to guarantee compatibility.
I don't really like the look of 8 buttons, but it would be more future-proof. I'd definitely make the last two black or a color that blends into the background, with the core six a bright color that stands out.