Having 18" wheels shouldn't have anything to do with getting the alignment right. Shouldn't make a lick a difference compared to a 15" wheel.
Now, radial tires vs. bias ply's changes the specifications you should set the alignment to. But even that shouldn't effect actually getting the car to drive straight.
For radials you should be using alignment specs for radial tires, not the factory settings (which are for bias ply tires). You should base your alignment specs off of this chart
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If you haven't replaced your suspension bushings, consider using the Moog K7103 offset UCA bushings to increase the amount of caster you can get because it's hard to get more than a couple degrees with the stock parts.
And yes, I have 18's and had no issues getting my car to drive straight.
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