Hi Rab,
I think most cleaners have a policy similar or the same as Ians myself included. Touch wood I haven't been called back or encountered a problem after doing a job.
I make sure to justify any possible problems (set stains, wear and tear, badly fitted / already damaged carpets etc.) before I start and as long as the customer is happy I proceed. If not then I walk away!
Our customers are the same as me and you when we pay someone to do a job for us, as long as we get what we've agreed then no problem, if not then there would be!
I don't think there aren't many people out there that 'try it on'. I have heard of a few cleaners coming unstuck but they're very few and far between. Just be sure to advise of any possible problems. As always, if in doubt, walk out

Kind regards,
Jason.