This is my point. How do you know? As I said, I've seen and been in loads of situations that have gone right to the wire and then backed down to nothing apart from some verbal

I think the trick is to genuinely not care or feel upset about hurting someone who is obviously aggressive towards you. If they encroach your space,and freely use verbal intimidation, the chances are they have got away with this numerous times. Its territorial pi$$ing, or ego inflation or whatever.
Its not your job to assess that- you are not trying to second guess them and see the bullied 5 year old within. Leave that for the social workers.

If you dont hit them, they will never learn that their behaviour has consequences. Working the door, you have to be calm and approachable, but you also need to let the punters know that there is a line that they overstep at their peril.
If you dont, you just make work for yourself.

If it nags at your conscience, look at it this way- the worst they are going to get from you is a sore jaw and a ride out the fire escape. If they try their hard-ass routine on the wrong person in public, they could get killed.
By hitting them, you are saving their life. its the kind thing to do.
Don't let the door hit ya' where the good lord split ya'