In my humble opinion you might as well let them watch! You already have their money and it is your skill they are paying for. It is not like you are training them how to do it. Even though it is not that hard, they'll still have to figure it out. Even a Chimpanzee would have to practice, anyway.. Since the information is *and should be* publicly available I don't see a problem. If the information wasn't publicly available, then there would be a problem. ..but I feel that the hiding of the information would be the problem. You may find that when you try to hide things, people just get more curious, and when you just do things righ out in the open they sometimes go unnoticed. It depends on the people involved, among other things.
Besides, people sometimes refuse to believe that their locks are vulnerable until someone shows them! If they are concerned at all about their security, then it would be in their best interest to watch and learn! The smith might sell another lock and some keychains as well. People respect things that can be proven, and people who tell the truth. If you can prove to them that their lock is garbage at least they know you are honest. It's a judgement call, but if they ask to watch, why not?
If I had to call a smith to open a lock that I couldn't and he/she refused to let me watch, I would either cancel, or not hire him/her in the future... If he charged me money just to watch, I would feel ripped off because I would be paying extra for something that most smiths around here do for free.
In my experiance, most U.S. locksmiths, at least here on the West coast and also in the midwest will just let you watch as long as you don't bug them. That is fair enough.
