As Greg said, what's important are the operating angles. You want them to be equal, opposite, and as small as possible (less than 3 degrees), but not zero.
That said, the type of rear suspension that you have will make a difference because the pinion will rise under power and thus change the operating angles. So once the angles are set statically, the rear angle will need to be played with in order to find the sweet spot under dynamic conditions.
As for tire size, it has zero effect on operating angles. What does have a direct effect on operating angles is the rear ride height, assuming that you are using a solid rear axle.
Andrew
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