I don't think I've ever seen a player get knocked off his spot ONLY a few feet by a vigorous block-out. To me, a vigorous block-out almost always puts an opposing player on his/her behind. That draws a whistle from me everytime...everytime I see it, that is!
Too many players these days have spent too much time watching Reggie Miller, et al, as they fall down after ever shot trying to get a call. If I see this, I usually tell the player, "Get up! You're not gettin' that tonight unless it's real!" That usually puts an end to it.
General rule of thumb for the coach: If he goes over the line but no one else hears it, I probably let it go...but he/she grandstands and others hear it, WHACK.