In reply to petegunn:
If you get a sore neck then just get some.
For belaying a leader, they can be more awkward, as most people want to watch them directly as they clip the first two bolts, then switch to the glasses.
You say you work at the wall which I assume mainly means toproping. There is far more margin for error in this case, and I'd suggest having the glasses on from when they leave the ground would be fine.
Another thing to consider against would be communication - if you are not looking at them, they may not hear you. Also confidence, they may not feel you are paying attention.
Also you would lose 20mins a day explaining what they are and letting everyone have a go!