Yes, of course I'm streaming MP3s from PC to Xbox. Used Relax with XBMP, but now I'm having flawless performance with XBMC and SMB.
Why leave my PC on? Well, it's a file server anyway, and is always doing SOMETHING (downloading, video encoding, etc.). Plus, why rip/encode to your PC, then STILL have to FTP your MP3s locally to the Xbox?