For car valeting businesses, it is always worth spending a few minutes checking the car over BEFORE you start valeting. Look for bumps or scratches or unsightly marks and take a picture of your phone. Of course if you are a valeting company that does not introduce swirls or scratches then your safe, but still worth checking.
Also, do not wear a ring or a belt with a metal buckle as you may accidentally scratch a car. Do your best to check out both the exterior and interior, it will give you peace of mind and save any unnecessary concerns with your client and keep your reputation as clean as the cars you detail.
As good practice AUTO VALETS always check the cars condition and if nessesary inform the customer about any marks on the paintwork, wheels or interior and that way at least you have dispelled any notion that you have been careless.