The more caster you can run without ruining your pinion angle the better. Caster makes things feel more stable on the road, thats why Mercedes runs at least 6 degrees positive. The less caster you have the more your steering will wander due to changes in the road surface. If you set the right side with slightly more caster it will compensate for the crown in the road, to get rid of a pull to the right. I don't know about the 2 degree difference suggested, seems a little much to me. With passenger cars, 1/4 to 1/2 degree was all that was needed. I've done over 10,000 alignments, with very few comebacks. Not blowing my own horn, just sharing my experience. Do what is best for your own circumstances.