I was looking over the BTM charts in the Jan 2012 edition(page 25) and for the three styles that bowled each has a different entry angles. The tweener seems to have the least amount of angle while the cranker had most. Four balls that I looked at were the omen, elevate, brick, and 811A. The crankers entry angles ranged from 7.05-7.9, the stroker, ranged from 5-6.6, and the tweener varied from 4- 5.5, but mostly in the 4 range. A lot depended on the pattern and the type of ball being used.
I remember BO Burton discussing 6 degrees in the ABC telecasts. I don't know if that how it got started or how scientific his statements were.
As far as the question is concerned tilt or forward roll should be used appropriately and the best players usually vary it when they need to.