LOL i haven't heard this one before either!
have heard the opposite worry.
but yep the girls are right - theres no proof that dummies effect teeth (or that no dummy effects teeth LMAO:P) - and same with thumbs - only if they thumbsuck until an advanced age (like 12) and theres still a chance their teeth will be fine!

my DD's both used me as a dummy and i was pleased with that;lol