I'm very wary of people flashing to let me out, they could be flashing at someone coming the other way or just trying to find their washer wipe. I am kind enough to let people out but try to anticipate how they'll read it and whether on not there is any other traffic that they might get in the way of. It's too easy to just be ignorant and rude but I treat people on the road as i would on the pavement. It is annoying when you let a car out in front of you and they then drive horribly slow.