For me, accepting or rejecting a review request depends on the type of book(s) that is being requested, the time frame (if there is any) that I need to have the review posted by, and how many other requests I have pending at the moment. Also, it depends on whether or not I am too busy with my own stuff and have enough time to read the book(s).
If the review request is for a book that's already published, I usually also look at the review ratings on Amazon and goodreads, and read the previews if there are any. That way I have an idea of what the book is like, which usually helps me make the final decision on whether or not I want to accept the request. I also look at some of the positive and negative reviews for the book. But I try not to do that too much though, because I feel that the reviews, especially the negative ones, might influence my thoughts before I even read the book, if I were to read it.
That's pretty much how I go about deciding which review requests I want to accept. How do you decide when you get review requests? I am a curious person, so I really want to know what you guys do to help you figure out if the book is something you might like to read or not. :)