I use the following rating system:

1 star: hate it or random noise - for crazy concept albums or songs I absolutely despise
2 stars: tolerable - usually has annoying noises or some low-quality song
3 stars: OK - not good, but not great
4 stars: good, but not my fav
5 stars: Excellent! or all-time fav

If I absolutely refuse to hear a song ever, I uncheck it in my library. Why keep them? I'm a pack-rat, I guess, and iTunes accomodates me!

As far as Jazz vs Classical, that's what Genre is for. My only wish is that iTunes had a 'Styles' field, similar to what All Music Guide uses (www.allmusic.com) - stuff like Garage Punk, Indie Rock, etc. all fit under the genre of Rock.

Wells here's my 2 cents on rating. Mostly my system breaks down like this:

0-don't like this song, should dump it
1-don't like but it's okay when listening to the entire album
2-Okay for random listening
3-Okay song, or it's a good one that I've played to death.
4-Love it, want to hear often
5-Top favorite, of course.

My rating system is as follows:

5 stars: "Best of Best" playlist

4 stars or higher + year : "Best of Decade" and "Best of Year" playlists

3 stars or higher + artist: "Best of Artist" playlists

2 stars: Remains checked in the library

1 star: Gets unchecked in the library

Using Beatles Abbey Road as an example:

5 Star = undeniable classic/work of genius "Because" "Here comes the sun" "Sun King"
4 Star = personel favourite but yet to earn classic status "Golden Slumbers"
3 Star = outstanding album track "Polythene Pam"
2 Star = classic album Everything else except "Octupus Garden"
1 Star - reserved as a temporary rating for later evaluation
0 Stars = Octopus garden