I went through a phase of refusing to thank people for following the law - and I still refuse to thank people who stop at a zebra crossing (because then I'd have to do the same for people who stop at red lights, etc) - but I agree that you catch more flies with honey, so smiling and waving at people for not being dicks is now my thing.
This,
Yesterday, separate occasions on narrow roads with no safe overtake. Environment agency van, driver patiently sitting behind me through around arm of twisty bits, got a cheery thank you hand wave.
Two utter morons in a Monday, on a far narrower, windy road with no real overtaking spots and poor forward visibility, but I knew widened out after about 4-5km. They were sat behind me after about 2km, revving and surging, including one aborted unsafe overtake attempt in a field entrance. Without the revving and surging I'd have pulled in, but by this point was feeling less than charitable and was doing 20mph anyway.
When they finally passed me, slow handicapping from the passenger, got another cheery wave from me.
I passed them stopped in a farmyard about 2km further up the road FFS