You could do either. Ginger can have wild yeast on it, so unless you want that in your cider, make sure you sterilize it.
You could get a test for that too, split batch, one sterilized, one not, and see what those little buggers will contribute.
I recommend adding pinches of sterilized ginger at a time to primary after ferment is done, let it sit a few days, taste it, and keep adding until you get your desired amount in there. Then get that ginger out so it doesn't continue infusing more flavor. Then do another batch with the unsterilized ginger in with your pitch. Wild yeast can be a lot of fun to play with.