It could go either way depending on how you bring up the cituation. If you just go and say: My coworkers haven't are trained bad and are doing a bad job that makes not only them look bad, but your boss look bad (he trained them didn't he?), and it makes you look bad. I would start out by saying something like this: It has come to my attention that my fellow coworkers are not treating the customers properly and are giving them bad service. I thought that this would be important for you to know so maybe we could fix the problem. (Make sure you say the second sentence exactly how I said it). By saying it like this it shows you care about the customer, care about your job and work evironment, it shows you have leadership abilities, and it shows that you are a good employee, this will look very good down the road when it comes time for promotions, raises, bonuses, etc.
Many Of Your Coworkers Are Not Properly Trained And Often The Customers Are Receiving Very Poor Service. How Effective Or Ineffective Would It Be To Discuss The Problem With Your Supervisor?
Have some specific examples when you go to talk to them. It wouldn't help much to generalize, and they wouldn't take you seriously. If you have examples and maybe even possible solutions, they'd be crazy to not listen to you.