I posted a question about string slap once on the "Form Forum," and Moebow posted a link to a thread where he went into great detail about the proper bow hand position. In the pictures he clearly demonstrates that the knuckles closest to the hand (or "punching" knuckles, if you will) should be at a 45 degree angle from the center line of the bow.
When I remember to do that, as well as have the elbow rotated in such a way that if I bend that arm the bow goes in FRONT of me horizontally and NOT up or down, I don't get slapped with the string.
That said, I recently bought another armguard because (all too often) I forget to do one of those things and slap the crap outta myself!
Moebow had the perfect analogy with the helmet statement. Or as they taught us in the Correctional Officer academy, regarding security equipment..."It's better to have it and not need it, than to need it and not have it."
Sorry for the long post, but I hope it helps.