When you load an mp3 to iTunes, do you still need the original file?

I download my music through other music downloading sites/stores and I only use iTunes to organize them and get the music to my iPhone. When I download these mp3's they are stored in my iMac's Downloads folder and I have a ton of music in that file. Do I still need these mp3's or can I delete the ones just floating around in my Downloads folder?

Look for the files here:
~/Music/iTunes/iTunes Media

If you see the files there, all is well. If not, remove them from the iTunes library, change the iTunes preferences > Advanced settings to "Copy to iTunes Media folder" and add them all again.

Click Edit > Preference> Advanced tick both the boxes
1] keep my iTunes media folder organized and
2] copy files to iTunes media folder when adding them to library.

if you have done this, you can safely delete files.
But its always a good idea to back up your music files and other stuff.
i have 150GB of Music, all backed up DVDs. I use iTunes for organizing my music, Music videos, ebooks, Audiobooks and PDF files [ generally eBooks in PDF format ]