As a writer, reputation ranks almost as highly as skill and novelty. We would never find the likes of Stephenie Meyer and JK Rowling rating a fellow writer's work because it's not in their interest to alienate both fellow writers and their fanbase/followers, who are potential readers of one's own book. From a PR perspective, this is a huge no-no. In fact, traditionally published authors are often encouraged to endorse each other's books and so widen their markets and target audiences.
Indie or not, I suggest never to rate a writer's work below four stars. If you can't give it four stars, then don't leave a rating at all because leaving a trail on the Internet is forever and someone's bound to find out sooner or later and might not take kindly to it.
To all fellow authors out there, we're writers and not reviewers. What you may not like, others might end up loving, hence, it's not in your best interest to insult any reader's (or reviewers') taste and lose potential sales. Since you have your own style and expectations as to what fiction should read like, you're not unbiased. By leaving a low rating, you're not helping your reputation in the writing community either because people will start talking about you, and certainly not in glowing terms.
Sometimes silence is worth more than gold!
To your publishing success!
