5 is perfectly fine imho, i usually get to 2/5 or 3/5 as cav, depending on how tolerant the people i happen to bump are. Sometimes they run into me, i bump em, and they report me...retards, but mostly upperblocking is enuff to prevent getting reported, at least in most of cases i met so far
This system is fine, griefers wont get out of it and it tolerates some sort of accidents, if it got reduced to three, it would be much worse.