I had the dealer under extended warranty, replace the pinion seal for a minor leak. I wish i had left it alone. I once had a quiet differential, after their repair the differential had a whine because they got the preload screwed up. It was a PITA dealing with them afterward and getting it straightened out..
My truck has a minor pinion leak, it has been leaking for over twenty years. I had to top off once during that time. Like I said, unless it is leaving a large spot on the driveway overnight, I would leave it alone. You may never have to top it off.