if all your your music is being copied into your itunes folder then your preferences are set that way. this is the default config for itunes. afaik if itunes is set up like this you can delete the originals with no effect on your playlists. the only problem that i could see with something like this is that the track(s) would appear twice in your playlists.
try getting info on a few tracks and see where the sources they are pointing to are located. if everything is in your music folder then deleting the original will have no effect at all.
The default setting when importing a song is to copy it to the /ITunes directory and is stored by artist/album. This is known as "managing" your songs I believe. The original can be deleted without any problem once imported to ITunes (since it's now just a duplicate copy).
You can turn this feature off or on. If you want to manage your own music in some other way (not by artist then album) you can turn it off and itunes won't copy when in imports, it will point to the original.