First of all, if you don’t know the difference between touching and groping, you may want to look it up. Some photographers seem to be very concerned about touching a subject, I am more concerned to keep models from touching me. Just joking.
From my experience, touching a model or any other subject can ease tension and build a certain level of trust. Of course, you should not touch someone just for the sake of the touching, but to move the hair or help with the pose or expression. It shows the subject that you are trying to make him or her look the best. The subject is not able to see what you see and what the camera will capture.
Also, you shouldn’t just go ahead and touch someone, but instead make it known that you will fix something or ask if it is ok if you would fix something. There is also no need to grab someone if the slight touch of a finger is sufficient.
Always remember: when dealing with people there is a chance for being sued for some bogus reasons, so be as professional as possible. I know that some photographers even have their models sign a form after the shoot, stating that everything was professional. If you plan on doing something like this, keep in mind that the model may be able to claim duress. Think about handing this form to the model after the initial contract is fulfilled to be on the safe side.
Models, there are many wackos out there who claim to be photographers, so be careful. Never be afraid to tell someone when they are crossing a line or whenever you feel uncomfortable. There is no reason for you to put up with anything that you don’t feel 100% comfortable with. When someone approaches you about a shoot, please make sure that person and his or her claims are legit. There is no way you could be a fashion model if you are under 5″8′ and weigh over 120lbs., no matter what someone tells you. Be reasonable about yourself and stay away from people making bogus claims.