Like 1stgen said, you can't just say "12:1 will be fine on pump gas" for every motor.

That said, I would stick to about 11.5:1 if all you can get your hands on is 91. That's what the Toyota 2ZZ-GE (Celica GT-S/Matrix XRS/Vibe GT) runs, and it's a production motor intended to be sold anywhere.