I had the same doubt....Uthscsa states on its website that it doesnt need one.UMich doesnt need one becos i sent my app without it and they sent me a mail stating that its complete. I sent to Upenn too and i talked to the lady there and she said my app was complete too.So, i think if they dont mention it, u dont have to send one.I did send a cover letter with every application.
HTH!