this is absolutely true in our business as well, we avoid doing work for lawfirms (rooftop or office heat and air) and homes of lawyers at all costs, or we high bid the job to double or more so they pick someone else.
also high on our list is restaurants of all sorts, chains included (mc donalds, etc), doctors offices, house flippers (not many of them any more), and certain ethnic groups will fight you just to fight you. not to sound racist, but if we do work for east indians, pakistanis, turks, or greeks (I know I have surely offended someone here) they will haggle you to death, and then haggle you AFTER the work is completed even if you have a contract, so we have to bill in an additional 20% or so, then when they haggle, we come down some (to our normal price) so they will be happy. I think it’s a cultural thing.
Also, some groups will be very educated when you do a service call or an install, they know what they want, they know the retail price of it, they know how long it should take to install, and they will pay that fair price (almost all Asians fall into this category), and we almost never have an issue with payment from them, once they agree to a price.