I usually don't respond, either, and I think that's OK sometimes; it really just depends on the situation. Sometimes, I get that 'doormat' feeling, and that's not good, but more often than not, it's just not worth it to respond in kind.
I guess it's about how you feel afterward. I'm glad that you brought up this topic. I think it's a good one for discussion of an issue that many of us have.
Warmly, Peanut
<font color=blue>HI FROM PEANUT</font color=blue>