Hair ties typically are safe for babies, but a few things to keep in mind with regard to choking hazards.  If your baby pulls their hair ties out, make sure to remove them before placing her down to sleep.  Hair ties with bows or plastic accessories can become choking hazards, make sure to supervise your baby while they are wearing them.  If your baby removes their hair tie, take it away and do not allow them to play with it as a toy.