Actually, the correct thing to do isn't to take the salesman's word for it no matter how the salesman is paid. The correct way to go about getting the best deal is to do the research yourself.

Not long ago, I was looking for a pest/termite control solution. I got quotes from various companies, some pushing liquid based solutions, some pushing "no spray" solutions, and some pushing the "Sentricon System". I chose the proper solution, not based on whether their salesmen are paid on commission or not, but based on taking an hour out of my day to research the things they were trying to sell me and see what the truth is. From my research, I was able to toss out the sentricon scam, find out that the other solutions were legitimate, and make an informed decision based on actual facts. That's really what needs to be done, you need to look at the arguments being made, not the person making them.