I'm sure a lot of my ratings were pretty arbitrary, depending on what kind of musical mood I was in that day, but it's better than nothing.
The way I did this was I made a smart playlist called "Unrated" with the rules: "rating is 0 stars; match only checked songs", then set the "Source" in Party Shuffle to the "Unrated" playlist. This will cause it to play random unrated songs until you've rated them all.
The rating scheme I used was:
|0 stars:||I have not yet rated this song.|
|un-checked:||I hate this song and never want to hear it again.||11%|
|1 star:||I think I don't like this song, but I'm not yet sure enough to kill it outright.||11%|
|2 stars:||It's ok. Don't love it, don't hate it.||33%|
|3 stars:||This song is pretty good (but not great).||26%|
|4 stars:||I love this song!||10%|
|5 stars:||An ass-kicking, incredible, all-time favorite.||9%|
That took about five months for somewhere around 19,000 songs.