I know the normal policy is the customer is always right, but, I have heard some clients before get it very wrong. (One in particular told me they used Dettol as a cleanser)
You can do an amazing treatment and they could be really happy with it at the time, but the second they leave the door you cannot guarantee that they will look after the area proberly.
Most of the time it is because your skin can feel a bit itchy after waxing, as we all know, it is hard not to touch it. Before the client leaves they are touching their bag, purse, money, door handle, car and I dread to think what they have touched before they come to the salon. That bacteria on your hands can be there all the time so you just have to advise your client not to touch it.
The best thing to do is have little info cards with aftercare advise on, hand them out to clients who are new to you and just very quickly run through it with them. Some clients who find their top lip very sensitive after, I give them a cotton wool pad with some aloe vera on to leave with. I tell them to use that if they need to touch it.
I hope that makes sense. I have rambled on a bit here. Sorry!