Pandora Filtering

Is anyone familiar with exactly how Pandora selects the songs to play on one of your stations? I ask because I never know sometimes whether to give a thumbs up or down to a song. For instance, I might like the song, but not the performer doing it, or even different versions by the same performer. For those familiar with the Grateful Dead, for example, some of the recordings from some of their live shows frankly sound like crap: they're out of tune and off key, while I may love the studio version of that same song. So if I give the thumbs down to a song like that, am I never going to hear any other versions of that song? Is there a way to go back and change a rating you've given, as I've sometimes hit the thumbs down by mistake?