Welcome to Cafe Libri! Great question and concern. I don't post my reviews on Amazon, but I do post them on Goodreads, epinions, Shelfari, and Yelp, and they all use a similar rating scale, 1-5 with no negatives. I haven't really laid out how the lower ratings translate from the other sites, but the way it works in my mind if as follows:
+5 = +5
+4 = +4 or +3
+3 = +2 or +3
+2 = +2 through -3
+1 = -4 or -5
I don't really see the ratings as grades like you do, but I estimate where the book falls based on my dislike of it; did I really hate ir or just a little bit? Most of the time my negative ratings come from a +2 score on another site, as you see I make that a wide range score catcher here on Lunch.
I hope this helps you. I'm sure other will participate in this discussion and offer their own advice about rating systems too that are probably more concrete than my own. One of these days I will need to solidify my rating system and check how my scores compare, lol!