Ive offered this for years and never had a real problem, although Ive had two who were so obviously trying it on, after I had done an outstanding job for them, that I just left them to it with the money in my pocket. They weren't going to complain later, they knew I had done a good job, but were perhaps a bit peeved that their con didn't work.
I have not charged a couple of times when I wasn't happy with a result, but these cases are so seldom.
I don't emphasise the guarantee verbally when quoting, but I know someone who does when potential customer hesitates at his high prices, and it is often enough, to have them book the job.
Dave.