Rabbits are meticulous animals and will spend a good part of their day grooming themselves and will use the same area, for their toilet. Being caged limits how clean they can remain and its up to you to help.
Keep their cage spotless!! Be careful of cages with a sagging floor that hits the tray below when the rabbit stands in the middle of the cage. It means he will be sitting in his toilet! My choice of disinfectant is IODINE SANITIZER
If you feed hay or straw be sure to remove any they don't eat within 8 to 12 hours so they won't soil it.
If they urinate on the straw or hay and then sit in it they can develop a terrible hutch burn or staph infection on the genital area which you might not readily notice. If they are shedding any parasites they will remain on the hay or bedding and may be re-ingested during grooming. I prefer using a hay rack hung on the side of the cage where they can pull only what they will eat into the cage.
Some rabbits are better housekeepers than others and will treat food as food and litter as litter so here again you will have to be the judge.
WHAT ABOUT A SITTING BOARD?
Many breeders recommend having a board or piece of sheetrock in the cage.
Placing a piece of plywood about 12 inches square in your rabbit's cage can serve three purposes. One is that it offers him a way to get his feet off the wire floor; two, it gives him something to chew on to keep his teeth worn down; and three, he can fulfill the natural urge to dig and scratch.
The inherent problem with solid boards is that many rabbits are not particularly good housekeepers. If the bunny urinates on the board and then sits in it he can burn the tender skin in the genital area. More importantly, if your bunny is harboring any parasites that are shed in urine or feces, he will reinfect himself when grooming his feet which have become soiled with his waste.
If you find your bunny it smart enough to keep it clean, fine. If not, put the board on edge and wire it to the side of the cage. He can still scratch it and chew on it without getting dirty.
Plastic bread trays are my preference for a resting board since the material is heavy enough not to sag and yet has an open grate which allows the urine and feces to fall through. Ask your local grocer to give you any broken trays he is planning to throw away. The trays typically break on the corners only which doesn't matter for your purposes. When cut in half the two pieces are a good size for large and giant rabbits or if cut in quarters, they are adequate for dwarfs or medium breeds.
There are now plastic trays available commercially for rabbits from a company called Extrona and E-Z Mats. See catalog for pricing