Pinion angle has guidelines and that's about it. We had a SS spring car that need 10* down to work.
If it's a dedicated drag car, turn it down to where it works best. If a dual purpose street strip car, you need to balance the two. Too much down, and the u-joint takes a real beating under normal street driving. It may cause some pretty serious vibrations as well. That's part of the reasons I like cal track style set ups. They don't require excessive pinion angles.