If you choose to backsweeten with a fermentable sugar (I prefer to use concentrated apple juice to sweeten, 3 or 4 cans usually sweeten to my taste and helps add more apple flavor that most people expect from their cider) use campden tabs at 1 per gallon and 1/4 tsp Sorbate per gallon. You must use both to safely stabilize your cider.
As stated previous you could be ok since you are kegging but.......You may want to pull off a growler to give to a buddy or I often bottle up the tail end of a keg to make room for my next brew. I just figure better safe than sorry, do as you please.