well it depends on the road and conditions... i'm going to assume that you're mostly talking about mostly lightly travelled rural highway type roads like what you see in most of Montana/Idaho...

if the speed limit is high, say 45 or 55, then slowing down is way cool. i toured the Pacific Coast and the cars zip by at 60mph more - once an 18 wheeler blew by creating a wind/noise combo that scared the pants off of me and almost blew me off the shoulder... it's only creepy if it scares the cyclist into thinking someone is going to mess with them or is hanging on their tail.

as to the honking, generally don't as it can scare or irritate cycliss. some drivers do the "honk to warn" you as they pass which usually justed scares you as it's like walking up behind someone and yelling "bo!". as to the after-the pass honk, i think i would get the message, but only if i were paying attention. if i were in the zone or chatting with some buds i might just think you were pissed that you had to slow down to pass annoying cyclists... just don't honk right before or as you pass and it's probably not bad and could be could.

if the message gets across that you're being supportive, this is almost always appreciated - like one of those "dunna dunna nah, dunna dunna nah" funky horns with a thumbs out out the window mabye 10 seconds after passing would be cool.

as to stopping... if you're on a deserted road on a hot day a ways from a water or food stop, i could see it being very welcome, but it depends as maybe the riders don't want to stop... i would suggest if you want to make a sign that you either hang out the window as you pass or stop way ahead and display a sign like "Free Water for hard working Cyclists" or "Water Break for Human-Powered Travellers" or something like that... then if they're busy they can just wave and thank you or can stop for a break - then it's the cyclists' choice. there are times in my tour when something like that would have been really cool.