I just finished slipping some 4 degree shims in my rear springs. I have a CV driveshaft, and the shims canceled all the angles out to less than a degree. I'm using a magnetic carpenter level and it reads between 1 degree and zero each time I measure. I've taken multiple measurements at the pinion flange (plate clamped to it), on the driveshaft and using sockets on each cap of the rear u-joint.

By now I'm fairly well convinced that I'm closer to zero than one degree off.

General practice I understand is to leave the pinion a degree low to allow a little bit of tip on acceleration, but what about when employing a traction bar as well? Do they tend to keep the axle located well enough that I can ignore the leave it low rule?

With a traction bar, you want to keep the alignment near zero degrees (w/ a CV shaft), assuming the bar prevents any rotation of the axle. If it is not 100% rigid (like if you have poly bushings in the ends), then you could leave a little up-angle.

I believe that part of the reasoning to leave a little angle in the joint is so that the rollers in the bearing aren't always rubbing on the same spot. If they have some slight movement then grease can have a better chance to move around and lubricate better.