wooo hooo! now i can listen to Busted without having to worry about my street-cred! I agree with what you say about listening to 'interesting' music to see what effect it has on your neighbours - online and in real life ;-)
The audioscrobbler plugin for winamp has a nifty feature that might help you... I'm not sure if it'll make it worth switching to winamp if you don't already use it, but if users of other media players scream loud enough, they might decide to add the feature for those players.
Anyways, the feature... In the Audioscrobbler Config dialog for Winamp, there's a preferences section on the top-right with a button labeled Include/Exclude Dirs. This button allows you to specify which directories you want audioscrobbler to include in your profile. Pretty neat...
Personally, I keep all of my new music in a specific folder so that it isn't reported to my profile until I decide to add it to my collection. When I play music from this folder, I use Winamp exclusively. For the rest of my collection, I use iTunes (though I could use winamp if I wanted). I don't really have anything that I consider to be a 'stain' on my profile so I don't have a section for it, but it would be easy to set up.
Hope this helps!
I had the same sort of problem. But I've realised now that it isn't worth my time avoiding listening to a song that I like. Seeing as I'm happy to discover new songs from any genre, my main concern was listening to songs which might have been classed uncool. However, I eventually came to the conclusion that I wanted last.fm to represent me accurately, rather than portraying me as I might have wanted to be seen.
It's not worth the effort of feeling embarrassed by what you listen to, or presenting a façade. My advice would be to stand up for the music you enjoy, no matter what it is.