Actually, haircutters and bartenders can make less than minimum, at least in some states. And, therefore, I always tip them 15-20%, depending on service. (10% if poor, 25% if exceptional, or even more if I received a discount, because you should always tip on the pre-discounted price.)
Other professions (hotel maids, food deliveries) are paid poorly, so I feel obliged to tip them. But some are tipped only for doing good service (furniture deliveries) or honest service (cabbies).
What are you supposed to tip hair cutters? I just am flummoxed on this.