Fk is an essential part of my service, it's rare I encounter bad breath. I have however learned to stipulate no heavy smoking, no more than a couple of units alcohol and no drug use close to a booking. This cuts out the main rancid breath culprits. If anyone kisses uuncomfortably I tell them. If they can adjust then game on, most do want to use a pleasing technique if they can't then I'll withhold but that hasn't really happened.
There was someone who's mouth was rancid with tobacco, I still managed it for that hour, but then brought in the no heavy smoking request which seems to have done the trick.