I think I've officially heard it all now!
The PHB should be pretty level because the more on an angle it is the more of an arc is travels through as the body rolls or the suspension compresses, and that in turn pulls or pushes the axle to one side or the other, and that's changing your track width.
Also a PHB is a part that must be there as it's the only thing that locates the axle relative to the body laterally. I think maybe lt1fbody355 meant to say that aftermarket PHB's are really only important on lowered cars with big tires.... I hope.
I'd state that better PHB's are a definite help to handling and stability and can benefit a stock height car that sees some cornering action as well.