It is a bad idea, but not necessarily because of lead, but because of electricity. What happens if your soldering iron is faulty? What happens if the circuit you're soldering is under tension or there are some caps holding charge?
I do this sometimes, but only when solder is in a plastic container, which allows me to hold the container in my mouth, without touching the solder.
I do this sometimes, but only when solder is in a plastic container, which allows me to hold the container in my mouth, without touching the solder.