I would want the buzzer working, for earliest warning that something needs my attention, not because of rules, but because it's a good idea?
I would also prefer for the warning to come much higher than the prescribed minimum....
If my coach's air system falls below 90 while i am driving it, something has already gone wrong, i don't want to be goofing off, enjoying the sights, all the way down to 60 lbs before something hollers "look at me!"
Higher setting alarms are available, just takes some looking to find them.
Happy coaching!
Buswarrior