D.G.
I would say to do it whenever YOU think...
My ped said with both my girls that it was fine and they both had it until they were 3. Before I had kids I would have thought that was outrageous to let a 2 yr old have a paci, but they only used it at night, gave it up willingly when they moved into their "big girl bed" and there have been no permanent effects that I can tell :) And yes, their teeth are perfect.
In fact, neither one of them were thumb suckers - a much harder habit to break.