I've already got this in my notesI believe there was an article somewhere with a step-by-step on how to prepare fleece to be used as bedding (so that it wicks properly) as well as cleaning and whatnot.
In my experience, IF your rats are not chewers and IF they are litter box trained and finally IF the fleece is well secured using fleece is great. I prefer it to wood or paper substrate. Use the 'hook' side sticky back Velcro on the bottom of the shelf. It holds the fleece pretty good. As long as there is no loose fleece, my rats won't pull it up, chew on it, or burrow under it. I also use puppy training pads under 2 layers of fleece. I haven't had ANY problems with odors as long as the litter box is changed out ever day.