Tipping is supposed to reflect your appreciation of the employee's service. If I need a cab and the doorman literally jumps to the curb, hand above his head demanding one of the taxis pull up, then helps my wife in by opening the taxi door, I will easily tip $5 or more because I appreciate what he did (and the rest of the night my wife will be talking about how great it was to have staff wait on her like that).
Conversely, if I detect a hint of attitude from the doorman, or they act like they have better things to do, I will NOT tip (this is rare but it has happened).
On average I find I am tipping around the $2 to $3 range for doormen.
Same rules for valet (but different amounts). So to answer your question, only tip doormen if you feel they deserve it, then tip an amount you feel is appropriate. And don’t ever feel obligated to tip if the service is below your expectations, for anything.