I dont think if you dont use a dummy your kids will have crooked teeth is true. My sisters kids didnt take to dummies and their teeth are perfectly fine.

Jesse uses dummies because at an early age all he wanted to do was suck otherwise he'd cry. Before i switched him to the bottle even though he was full of milk he'd just like to sit there with my boob in his mouth for comfort. He now loves his dummies so we let him(just not at night otherwise id be up every 5 minutes when he drops it!).
But it is totally up to you if you want to introduce them or not. If Julia is happy without them, then dont bother, but if she is constantly grisly or something then you may wish to give them a try. She may not even take to them if you do offer them, the same as my sister's kids