Oh yes I'd recommend to reply. It's something vital to stay in contact with Google about just like anything concerning your account!
I got one warning on a site that had the adsense word in it. I emailed them at once and asked why as their email just said it violated tos and the adsense ads were disabled. Shortly I got a nice email that explained why as my domain name contained a trademarked word, adsense. And how to use that domain, naturally without adsense, so as not to have google all over my butt. Learned quite a bit from that email! Second (and hopefully the last) I got, when I replied, they explained to me in detail why one of my sites had the adsense ads disabled. Basically, they did not like the "Keep our site free" above the adsense ads. So yes, do reply and ask them in a nice way what the problem is. Chances are they will reply giving you the real reason so you can fix the problem and make google happy again with you.
Replying is always the smart thing to do. It shows you care and are as concerned as they are unless of course you decide to be irate with them and completely unprofessional.
So if you recieve a mail saying your acount has been banned, then you can just fix the problem* and they'll allow you to use adsense again? *if the problem can be fixed. Copyright pictures, "Keep our site free" and so on.