Trimming the bumper endcaps should only apply to running 33's, not 31's. I ran 31x10.5x15 tires on stock rims for a while without any modifications at all and they never rubbed.....ever.

I've been running 33x12.5x15 tires on 15x8 rims for about a year and a half now. My bumpers weren't an issue because I swapped both front and rear bumpers out for a set of don custom bumpers. Granted, I have a 2" body lift, but the word is that even without the lift they shouldn't rub (I did my body lift and tire swap at the same time) except for on the bumper end-caps. The only thing which ever (briefly) rubbed was the splash-shields at the front of the front wheelwells. You can easily bend them a little bit forward with your hand to correct that problem.

But 31's on stock rims rubbing? Nope, they won't.