It depends on the situation but I think it is better to avoid the horn as much as possible as many motorists definately unnecessarily use it.If the cyclist is not as far right as possible and you have to pass in a tight spot a light warning tap may be helpful and slowing down is appreciated but sometimes motorists can try to be too cautious which can actually be more dangerous for all involved.As far as stopping I again think it depends on the situation and how positive of a reponse you get initially from them but keep in mind cyclists are going to be somewhat put on guard by vehicles stopping in remote areas so I guess I would err on the side of not stopping unless it is at a populated spot or you get particularly warm greeting back from them initially.Edit As I now see your talking about tapping the horn with the thumbs up after you pass I do normally appreciate this as it happens so rarely lol .