This will not be a long post, but I just had to rant for a moment.
When someone holds the door open for you, don't you say thank you? When you drop something and someone picks it up for you, what do you say? Thank you. When a blogger takes several hours out of their life to read your book and review it for you, what do you say?
At least that's what you're supposed to say. You'd be surprised how many don't say anything at all!
As the Masquerade Crew review manager, I personally deal with all of the authors we review myself. That means I accept and decline all books for review, post reviews to Goodreads and such, make sure reviewers get the books they request and communicate with authors when the reviews go live on our site.
It's just funny to me how authors are all gung-ho in the beginning when I first accept their request, then as soon as that two-star review goes up, you don't hear anything from them. And lately, the Crew has been giving quite a few bad reviews, so you can imagine how quiet my inbox has been.
Don't get me wrong, negative reviews SUCK. But regardless, good or bad, an author should still thank a reviewer for taking the time to read and review their book. There is really nothing else to be said about that, besides you're a rude JERK if you don't.
The authors I deal with can't say that they didn't know the review went live on our site. Not only do I email them to give them the heads up (not every single time but 90% of the time), but we use author Twitter handles in our titles so when our readers tweet our posts, the author knows it. So there is really no way to say that they didn't know the review went up.
Anyway, my point is, be grateful, regardless of the review/rating. The only exception is when a reviewer is simply bashing your work just to be bashing it and doesn't offer any worthwhile insight into why your book is being rated so low. At that point, it really is best to keep your mouth shut.