I would say if she seems normal in every other way, this could just be a quirk of hers that she will grow out of. I wonder though if her tongue has been examined. I would assume it has since you asked your ped about it, but you never know so I thought I'd ask so you can make sure.
My daughter was born with "tongue-tie" as it is commonly called (I forget the medical name just now) which could possibly cause a baby to jut her tongue out of her mouth as you describe. This is because the little piece of skin under the tongue is too long and keeps her tongue from moving freely. It can cause feeding problems. It doesn't sound like your baby has this, but thought I'd mention it in case so you can check. It is fairly easily rectified if you do find this, so not to worry too much.
Good luck. There are so many things for us moms to worry about. Your daughter is probably just experimenting to keep you on your toes and in preparation for all the worry yet to come!