I rented a Motorhome last year and got tired of waving back at all the other Motorhome drivers. The rental company never warned me about it.
I imagine there was a time, 30 years ago, when you may say "Look, someone else in a Motorhome, let's all wave", just as Volvo drivers used to acknowledge each other, but seriously...when you live near the A303 (the route to Cornwall) you can get a repetitive strain injury.
Whilst cycling on local lanes I always nod, wave or "morning!" other cyclists. I wouldn't bother on a regular cycle commute, to other obvious cycle commuters though.