my opinion on this situation is that I always do the best job possible, if a customer were to ring me up and complain and ask for a re-clean then I would make a judgment call and ask myself.........
would cleaning it again make any difference and the truthful answer would be no...... because if it could have come any cleaner then I didn't do the job properly in the first place.
so then i will do either....... just tell them I made the carpet as clean as it can possibly be made, but I want them to be happy so the best thing to do is make the clean complementary and give them a refund.
With some customers I know when I have done a good job no matter what they complain about and they have got good value for money so they get no refund.
But if a customer is taking the pizz then my attitude is like they are thieves trying to reach into my pocket and steal they money i would use to feed my children