The only time i have seen anyone nod was at a stop light, most of the time i am parked & someone who is also parked will say something to me or i to them.
Harley riders raise their 4 fingers off the handle bars at other Harley riders, some give a short wave or nod.
If your out in the boonies its smart to acknowledge any bike rider in some manner, you don't want to be 40 miles from civilization on a back road & need a wrench that was left out of your tool kit.
Sort of like if your driving & see someone broke down out on a desert road, you may not stop & offer assistance the first time you see them, but you do if they are still broke down when you make your return from where ever you were going. If your out on an unpaved road, you stop the first time that you see them.
It is all about recognition around town & survival out in the country or isolated areas