The dog is hers. Julie is a real noun. She gets an apostrophe. You use one when you squash words together. If you make a couple words contract, you make a contraction.
Contractions always get apostrophes.
You have two CDs, not two CD's. Didn't know this. But you can know that this time they are wrong, and are the ones who look dumb, and you can be the guy pointing your finger and chuckling at them. Which is pretty much the way government rule-making works, eh. Mistakes set in stone.