Lately I've decided to force myself to be honest when I rate music (like/dislike) on Pandora. The rubric is "do I enjoy listening to this song enough to want to hear it again?"
There are a couple interesting phenomena I've noticed.
First, initial impressions are not always accurate in the long term. I find myself hitting "dislike" for songs that I did initially like several months ago. There are probably some songs I disliked initially that I might like now, but of course I'll never know about those.
Second, and this is where the honesty comes in, is that I have had to admit to myself that I don't necessarily like music I "should" like. For example, I don't like any Velvet Underground song. There, I said it. They sucked. I also dislike most songs by the Jam, although I do think "Eton Rifles" is a good song. I don't like the Cure or Depeche Mode at all, which is somewhat surprising considering I'm a huge Smiths fan. What is harder is hitting dislike for artists I admire. There are Morrissey songs I don't particularly care for (anything off of Kill Uncle, for example.) I've considered hitting dislike for remixes of New Order songs (although usually I just use the "bored" feature for those.) It pains me to have to hit dislike for any song by the Mekons, but their most recent album (Natural) is, well, I hate to say it, terrible.