I've been guilty of this many times and to me it seems understandable as long as you're not overly aggressive or clearly looking to start something. Bikes generally don't have horns and our only way of communicating with other people is hand gestures or our voice. Just be careful because they're in a car, with a steel cage, seat belts, airbags, etc and you're on a bike. I've encountered instances of road rage where you just have to let the other guy go before something bad might happen.