Lens manufacturers say that anything other than a microfiber cloth will damage the coatings on a lens. But microfiber cloths eventually pick up crap and they look like they're a pain to wash.
How do you keep your lenses clean? Are you using cloth/kleenex or microfiber cloths? If you use microfiber cloths, how do you clean those?
I do that sometimes. When the lenses have grease or salt from the beach, and I have no quick access to water, I'll lick them. I don't recommend it, but I do it.