Additives in general should not be required and might even cause a little harm if used constantly (like, every tank). I do use a little once in a blue moon (maybe annually, if that) and only products from reputable companies. The two I prefer are Red Line and Stanadyne. Never use ATF!
Biodiesel is neat stuff if available - I didn't know of any stations with it here in Boise, until I tried Tim's link! Hmmm, that's not far from me, I might try a tank:
http://www.biodiesel.org/buyingbiodiesel/retailfuelingsites/showstate.asp?st=ID
