I normally do Tina's "Come On!" taunt after I win the first round, and then her kiss taunt if I win. They just seem so perfectly made for that kind of thing. Thinking about it, I should do here fist taunt at the beginning of the first match.
If I'm fighting in a bad connection then I tend to get a bit taunt happy if they just spam the same move.
There was a similar thread over on Shoryuken about being taunted in MVC3, and people got really up tight about it there as well. I don't see taunts as a problem. They're what they are: taunts. They're supposed to get you fired up and lose concentration or think "Oh, really? Come on then." If you get angry about them, then your opponent's already got the advantage that you're not thinking clearly. You'd be surprised how often this works on lower ranks (of which I'm only a D- in this game, but DOA4 I was A and I saw it happen then too) especially. It's like you're hitting a rage button and they stop remembering how they were going to play the game and fall right into traps.
Just because someone stands still after they KO you, doesn't mean they're trying to be hard or have a Samurai Spirit. It just means they can't be assed to do it. I'm sure you've taunted people after you've KO'd someone.