I can see where the OP is going here...
I try to wave to everyone i see on a bike, whether on the trail or on the road. I find that most ppl wave back. (Even roadies, in kit, sometime wave back.) To be sure, folks on the trail are friendlier than ppl on the tarmac. But, as many ppl said in some less friendly ways, it's not the end of the world if some DBs fail to wave back. And, really, if they're that type of person, you're better off getting snubbed, anyway.
The dude above who was talking about his heartrate and the effects that being friendly would have on it was probably being sarcastic. (I hope so) Still, many ppl do feel that way, basically. And you know what? I'm super-glad that DBs who worry about their heartrate on the trail seldom talk to me. (And, ya know what? talking whilst riding might actually increase your heartrate.)
But, yes, when you ride a motorcycle, most other riders will wave to you, regardless of what you're on versus what they're on. And, while most cagers don't wave to one another, jeep wrangler ppl will wave to one another quite often, as will most ppl in older VWs. It's a community thing, and it's totally dorky, but it's snobby and hateful to snub ppl doing it.
My policy? I wave or give "the nod" to other riders. If other riders say hi or wanna chat, i return the favor (unless I'm late for work). If there's a rider down from a crash or a mechaniocal, i will stop and offer assistance. Even if it's a heartrate guy. We're in this thing together, and we need to act as one. That sort of community can help protect our rights on the road, or our rights to trail access, etc.
-rob