A 10 mm spacer will likely require a hub stud change (to longer) so that the nut engages enough thread to keep the wheel on the car in the event you load the assembly enough in say a very tight corner that generates a strong leverage effect on the wheel. If you have bad roads put enough tire on the wheel to reduce the chance that it will be damaged in the event you hit a good size pot hole or step in the road.
In my experience with 18s, consider 225/45-50, or 235/45/50 rear only. I plan on switching to 215/ 50 on the front if I can find them because roads that hold puddles have caused some scary moments with the 225s on the front of the tale heavy Fiero when hit at speed. I have 245/40 on the rear.
[This message has been edited by Joseph Upson (edited 12-04-2014).]