Hold on Roger. I think he said the oil fill plug, not the pinion nut. Do not torque the fill hole bolt to 90 ft/lbs or you'll strip your case for sure!
If it is leaking, yes you should replace the washer, only a buck or two. It should never really leak much out of the fill hole since the level is supposed to be at the bottom of the hole. As long as you have the fill bolt removed, you might want to drain and replace the oil (you might have water in
there if it is overfilled). I like redline gear oil.