I personally have run two different sizes on both front and back, now I have 245 40 18 front, 275 35 18 back, I had 275 35 18 front and 315 30 18 rear, front was too wide stuck out a good 1/2 inch past fender, I liked the rear but rubbed on the outside lip; I had to bend the lip up. I like the way my setup looks right now, But I will be changing the rear to 305 35 18 next time. The 245 in the front fit perfectly just inside the fender.
FWIW, I have 255/45's up front on my 17x9's, and they look too narrow. I might run 275's next time. Out back, with those 10.5's, I would run at least a 285 or so. Shoot, I run 315's on my x9 and I think it looks real nice, probably not very effective, but that's why I'm building my rear suspension, lol.
i was messin around on tirerack to see what they suggested for 18x9 and 18x10. for the fronts it says 245/40/18 and the rears 285/35/18.
im lookin lookin around cuz im in the same boat and my car is lowered already.
btw dont wanna hijack, but heres a pic of the rears i just got today!