I've heard many times on here if you go with 17" wheels or larger, over stock 16's, you will need to move the rear bumpstops inward and grind the flange down. Why?
I just installed some 18" C5 wheels on the back of my car and I do not see how the bumpstops could hit the wheels unless I was to do some Duke's of Hazard stunts that would cause me to get airborne and let the axle drop enough to hit them.
I'm not lowered and my suspension is completely stock. Will I encounter a problem if I lower the car. I plan on lowering, LCAing, adjustable PHR, and relocating, the rear suspension next. Is this where the trouble will start?

 

If you jack the car up, the wheels will hit those stop plates when the suspension droops.

Also, if you are in a high G turn, the inside suspension point is extended rather far. With R compound tires, you could come very close to lifting the inside tire, especially under trail braking.

 

It's more an issue with the wheel width and backspacing.
For example, I put 17 x 11s on with 7.75" backspacing. That pretty well fills up the wheelwell. If I lift the car from the body, the wheels would catch the bumpstops before the suspension was fully extended. So I ground the stops for clearance.
A 9.5 inch wheel would never have this issue.
Mike

 

It's the upper lip of the wheel that you have to watch only if it's on a lift or if you jack it up, like 1Z2Z said, some place other than the rear end. Actually, if the car is lowered, that moves the stop and mounting bracket down away from the top edge of the wheel.

On mine my axle would have to drop about 5" or 6" (I'm guessing) or more to hit the wheel. I'll measure this weekend.