If there is something to reply to, I do reply. The way I look at it, if something is said within the review that is unclear/implies a question on the part of the reviewer, I figure 'hey, what could hurt? Even if s/he wasn't asking a question in a review, another potential reader could look at it and not understand it'. I like to give my potential readers something to look at so they know they aren't wasting their time with my stories.