You will want to handle them as much as possible right from the start, but you will also want to gauge your handling of them on Momma's behavior. If Momma trusts you and doesn't mind, then handle them frequently now. Don't wake them up if they are sleeping or disturb them while nursing, but if they are up and commando-crawling around, then by all means, scoop one up and get right up in it's face - blow on them gently so they can get your breath scent, talk to them gently, hold them against your heart so they can hear and feel those vibrations to better identify you as a friend.
But - if Momma is anxious, upset, meowing loudly and in a warning stance, then do not handle them further or you may subject yourself to a nasty wound. If she doesn't want you to handle her kittens, then don't for now. You will want to get them socialized soon though, so it might be best - if Momma is reluctant to let you handle them - to confine her in another room for 10 minutes at a time while you pick up and interact with the babies. Do this once a day for the next week or so, then try to let Momma stay around to see what happens.