I've read from several sources that spit should not be used because saliva contains compounds that are meant to break down food, and will act in a detrimental fashion to the polish. So I just use filtered water when polishing my shoes. I keep a small bowl nearby and apply a few drops as needed. I would think that the tannins in tea would be, as you surmised, detrimental as well.

Breathing on the surface should be OK, though, since it's just moisture with no saliva component.