I personally dont use racks but I have done a lot of reading around and I would have to say a lot of people love to use carefresh. I use carefresh bedding for me and my girlfriends hamsters, and it's so easy to cleanup, and it eliminates odors. You can easily tell if carefresh bedding has been urinated or pooped on also lol. Probably the easiest to clean and most comfortable bedding. Cheap also.
EDIT - in my ball pythons tank I use cocnut fibre for a more natural look, but for racks go for carefresh.