I have a Mach 1 and of course the rim size is 17X8. Since I lowered the car I would like to leave the17X8's up front and run 18X9's in the rear and issues except rotation?
If you are goign to run 1" larger rear wheels, you might want to look at runnign rear tires that at least 1" larger than the stock front tire size (or what ever size you are running up front).
Having a tire in the front with 4" of rubber between the road and the rim and having 3.5" of rubber between the road and the rim in the back looks like crap IMO.
If you are running stock up front, you should run at least 275/40R18 out back IMO.
I have 18x10 Deep dish bullits in the back with the factory 17's up front. I run 275/35/18 Kumho MX's and 245/45/17's up front, with the rim being an inch bigger in the back it actually does not look that bad,i like mine the way it is, not a big difference as long as you ahve some springs on the car.
the tires/wheels should be larger in that back. at least thats what looks good to me. Go 10.5 on the back if you dont care to rotate them, looks very nice.