A grease fitting is a good thought and easy enough to install. Even though those bearings are'nt spinning like a wheel bearing they do take a shock load from bumps etc. probably good to keep an eye on em

 

I just did mine last week after over 25k but only since I had the trees off polishing them. There was little grease on them and only a bit of rust (HD must use baby oil or vasoline instead of grease) so I repacked with marine grease.
I will add that putting a (singular) grease fitting in would only grease one bearing if that with the way the cups are made and only if it went in the bearing cup. The steering tube is wide open and exposes the joints where top tube and lower tube are welded in and would result in 90% or more of the grease from your fitting filling the void spaces and the frame and never reaching the bearing.
Once you put a good amt. of marine grease in there it will be good for a very long time.