We all have different styles and I get that. Sometimes it is a discrete nod like a gangster before something goes down - at least is is something. Other times it is an overt wave (up high or down low) - an acknowledgment from one cyclist to another. You are both out on two wheels for reasons only you know, but hey, give each other a little love no matter what.
I am reminded of the old bit in the Seinfeld series. Regardless of the circumstances, Jerry was always expected to say hello when he saw a relative. In the following scene this is demonstrated when Jerry is accused of not saying hello when confronting his Uncle Leo for shoplifting.
It is no different in the world of cycling. I would propose that regardless of what you are riding, where you are going or how many watts you are producing, you still say hello.
I recently was on a road ride with a friend last weekend when we passed another cyclist riding in the opposite direction. We both waived from our road bikes, but received no acknowledgement from the other rider - you still say hello. It is not as if he was distracted or did not see us - we passed on a 2 lane road miles from anything but the occasional cow, but he just stared ahead and did not even flinch at our passing.
Perhaps it is a misguided sense of brand loyalty if the opposing rider is on a different make of bike. Or maybe some are not comfortable with their body image and do not wish to draw attention to themselves in Spandex. The lack of acknowledgement is perplexing to those who are not confused by brand, road, mountain, cross, urban etc., regardless you still say hello.
Say hello! You never know, you could be encouraging someone who hasn't ridden since high school and got the guts up to dust the old bike off that has been sitting in the garage for 10 years. If you don't they may think that cyclists are jerks and snobs and put the bike away.
However you do it, just say hello.