I offer the same and have never had anyone try it on. You have to advise of any possible problems in advance verbally (if you trust or know them) or in writing (preferrably) if you think there could be a problem.
During surveys I'll give my honest opinion (possible limitations of clean, any problem areas etc.) then its upto them whether i go ahead.
I've done surveys where custys are torn between cleaning or replacing. Occasionally I've recommended replacing if they're too far gone or think results will be unsatisfactory.
If not I'll say I'll clean them and if they're not happy dont pay, cant say any fairer than that. Never walked away unpaid!
I think everyone should offer the same guarantee, I know a lot do already.
Would you pay a tradesman that did a bad job for you? I wouldnt!