bolt upright is great for motorways etc, angled is great for cornering, the balance for tyrewear and handling is normally cambered in slightly so that when cornering the tyre doesn't go to positive camber, thus promoting slide. I remain unconvinced by spirit level approach (except as a rough setting after a rebuild) as you are gambling on your body work etc being aligned 100%. having the job done on a proper rig usually entails the measurements being taken from the hub centres, guaranteeing a true setting (assuming the adjuster knows what he is doing).