I'm not in the U.S but when I took my emergency medical lifesaving class here in Denmark (too long ago to really be useful anymore) the guy doing the class said that there are a limited number of ambulances in Copenhagen and they will refuse to do some things if they are in use (at the time I think it was 10 ambulances) I would expect it is the same everywhere - if you are not in immediate danger don't use an ambulance because the number of ambulances and medical technicians are calculated to handle the number of emergencies expected.
then again one time I helped an old lady when she biked home drunk, fell over and got a concussion and they sent an ambulance.
However in my experience the incentive of the hospital in the U.S is to bill you as much as possible.