Depends on what you are hanging up - with this LO we are hanging most of the clothes and baby hangers work great with onesies and clothes with long sleeves. Baby hangers are smaller so not as much hangs out the arm holes.
You don't have to but some items you may find difficult to stretch on a big hanger.
Definitely use the baby hangers!
BUT I don't think I've bought a single one for DD1. I just ask to keep them every time I buy something on a hanger. I don't think I remember ever being turned down either. I've accumulated quite the collection over the last 2 years!