SansaMonkey is a tool especially designed for Rockbox ratings, you can delete low rating tracks too:
Features for the Rockbox Firmware:
- show Ratings
- export Ratings to *.csv
- transfer Ratings to MediaMonkey
- increase Playcounter in MediaMonkey
- import Ratings from MediaMonkey
- convert Album Arts (jpg, ...) to *.bmp (for Album Art Patch)
- transfer Play History to last.fm
http://www.mediamonkey.com/forum/viewtopic.php?p=87677#87677