After a recent experience of mine, I would caution against even that minor amount of interaction with any motorist. Here's my story:

On certain popular bike routes in my area, there are some car "enthusiasts" who frequently drive agressively and recklessly up and down narrow, twisty roads at high speeds, sometimes in groups. I would frequently encounter these drivers and, like you, thought that looking over at them a little extra would let them know that I see them and don't approve of their behavior.

One driver in particular, who I would frequently encounter, found that even this minor interaction from a bicyclist violated some unwritten rule. After I "gave him the look", he hit the brakes, made a U-turn, pulled up alongside me, rolled down the window, and yelled "Why you giving me the hard look, a**hole?". I did not interact at all with him during that incident, but encountered him numerous times after that where he made it clear that he had not forgotten me. He would pass me and then pull over up ahead at the top of the hill waiting for me to come by, and then tailgate me down the hill. Or he would buzz me while leaning on the horn. The guy abviously had some serious anger management issues as well as who knows what other personal problems going on.

After a few of these incidents, I completely avoided the roads where this guy routinely frequented. After not encountering him for 2 months, he sees me on a completely different road, pulls another U-turn and comes at me at high speed. Fearing for my safety, I duck down a side street and hid behind a garbage truck. He wasn't able to get close enough to accost me and ended up making another U-turn and heading back in his original direction. I wasn't sure what he was going to do if he caught up to me. It was either spew some more obscenities at me or maybe shoot me. But I wasn't taking any chances and went to the police. His actions were clearly "criminal stalking" according to California Penal code section 646.9(A). I didn't know what to expect from the police, but the officer did listen to my story and, although not taking the step to go out and arrest the guy, did offer to mail a note to the registered owner of the vehicle with his business card enclosed and ask him to contact the officer. I don't know if that action was enough to get the driver to change his behavior, but I haven't seen him since, although I have not ridden on the roads that are his stomping ground since then and I am extremely vigilant when out riding to check every car coming and going to make sure it's not that guy.

So bottom line is that I suggest you think carefully about even looking at other drivers as you may end up in a situation like mine where your are forced to give up riding on your favorite roads and fear for your safety on almost every ride because of some psycho driver who took exception to your just looking at him.