I touch when I need to and i always ask. I have a good rapport with my clients so its not a big deal. They know im looking out for their best and trying to get a good look for them. I think guys worry way too much about this . Its not that hard to say, can i touch the side of your face so i can position it better. Or place your hand in the middle of a back to emphasize better posture. Ive never had a problem with any of my female clients.
But,,if you start putting your hands on her a$$,, yeah,, your gonna have a problem. If your a photographer and your getting your jollies by touching your clients,, you should be shot and hung as far as im concerned. just use common sense folks and be appropriate with your female clients.