My little one has been sucking on his thumb every once in a while lately. He's 6 months old and exclusively breastfed. He has two teeth and I think he's working on a couple more. I'm not too terribly concerned because he doesn't do it all the time--usually when he's in his carseat and falling asleep. I keep hearing about people who let their kids suck their thumbs and how it ruined their teeth. Does this have any validity?