Yeah..good question. Personally, I don't flash my headlights as often as I tailgate. However, I would only tailgate for short amount of time, say, 30 seconds or so. Longer than that, either I give up or try to pass on the left lane.
Flashing the headlight is definitely safer than tailgating, but maybe it seems a bit rude? I donno...giving signal might be the best way, although through what I've seen, most people ignores them for some reason. Maybe it's because a lot of people seems to forget they signal light is on (or lazy to turn off....:stupid:). Quite common to see a car in middle giving signal to the right without actually moving to the right, ever...