I always find it's most embarrassing, no matter what I am driving, to actually use the horn -- the horn is just a way to loudly proclaim that you haven't been paying attention (your surprised by someone's move) or to say "hey, my time is more valuable than yours and you're wasting it!" (observe others if you don't believe me). I do sometimes give a quick "double toot" to alert someone else that they haven't been paying attention, though -- just a polite "hey, the light's been green for 10 seconds now, any plan of moving on with your day?".