This can provide flexibility in terms of ratings: you are not limited to five (or six, although I use no stars as 'unrated') discrete ratings. When you sort by rating, iTunes will take into account the actual value, not the displayed number of stars. The AppleScript code required to set the rating of the currently playing song to 95% is:
tell application "iTunes"
set rating of current song to 95
end tell
Note that values less than 100 will display 4 stars, even if they are greater than 90 -- values are rounded down to the next lowest multiple of 20.

