One thing however you got exactly right though, you asked on here first so that shows you were unsure and decided to ask, it does seem like common sense, even to people that are not carpet cleaners that it's unreasonable to expect a guarantee of removing a stain, can she guarantee removal of stains on clothing when and if she ever uses a washing machine, of course not, it is probably the one most important thing to remember, never guarantee to remove any stain, so when it does come out they think your amazing, when it doesn't they still think, well you said it may not come out.
Remember as others have said, they are calling you as an authority on what you are being paid to do, of course we all have to learn though but at least sound confident, use the forums for the wealth of info they contain and if you haven't already get some training under your belt.