Wave at anyone you feel is deserving of your respect on the road.
Here is my list:
Riders (SS) 85% wave back
Bikers (cruisers) 50% wave back
Scooters (sometimes) 10% wave back
E-Bikes (sometimes) 100% don't even notice
Classic cars (they get a thumbs up) 25% acknowledge
Jaw dropping litting kids crossing the intersection in front of me, slowing their parents up as they stare (little kiddy wave) 100% satisfaction
People motioning me to pop a wheelie (receive the "no" head shake)
Trikes (dont make my list)
People who wave to me (matching wave returned) 100% of time (nod substitutes of clutch hand is busy).
Sometimes all the waving gets to be too much, or in places where there are bikes everywhere (Europe) waving becomes restricted to...same make, same model, or when the bike frequency drops to 1 every 5 minutes. Some downtown core areas have a bike / scoot every 30 seconds. Waving loses it's cache in such circumstances.
As a rule of thumb and respect....wave back to those who wave to you.