I don't think 18's would throw the suspension geometry out at all, provided you get rims that are narrow enough (I think 8" is the widest that you can go from memory). If you lower the suspension then this will in fact change camber settings more so in the rear due to the suspension set-up in the rear, however if you are going the expense of 18's a set of lowered springs/coil-overs then the extra money spent on a camber kit for the rear is only a little bit more.
