I am buying some new wheels for my '12 JKUS. Will an 18x9 with -12 backspacing cause the wheel to stick out from the fenders? If it does, I'm fine with it. I just want to know what I should expect.

 

That sounds like offset, not backspacing. You will need around 4 inches of backspacing, not offset. Study up and find out what you are really buying first because a -12 offset will not give you enough backspacing. Study this link to see the difference:
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

 

Thank you buddy.