The "rear lower forward and the rear lower rearward" are referring to the control arm on the rearend, not the front suspension. The bushings on the rearend lower control arms are different on each end. The offset upper control arm bushings can be used to get the alignment in the ballpark and to help correct a caster or camber problem and then use the lower control arm cams to dial in the final alignment. These caster and camber problems can arise with stock vehicles as well as S10 upgrades. Every car is a little different so your mechanic is correct that it will depend on how it sits after it is together and sitting at ride height.
Clyde.........75 Monza 2+2