Generally speaking, slightly more pitch down on field guns and for folks with big &/or round fronts. Too much can cause facial slap, particularly so if the gun is being shot pre-mounted. Speaking in inches up from the floor, that translates to .5~1.5" away from the jamb 26" up as the normal usable range. If you do a lot of mount & shoot patterning you can prove to yourself that there is a tendency for the gun to shoot a bit higher with less pitch down. Negative pitch or pitch up can cause a gun to slip from the shoulder pocket. FWIW, purpose built international skeet guns will almost always have more pitch down than purpose built skeet guns for American NSSA targets.
My own experience has made me of the school that pitch does matter, but its goal is to do what Jim says, make the gun fit [contact] your shoulder pocket as fully as possible. I would add 'without introducing too much of another component'.
