You definitely want to cover the grids with something. Usually with wire floors, the spaces are no bigger than 1 by .5 inches. The holes on an NIC grid are about 1.5 by 1.5 inches. It is much too large for a rabbits feet on a long term basis.
You can use coroplast to cover them. Just cut it to the right size, you may want an edge going up along the back to help keep poop and stuff inside the cage. It may be helpful to zip tie it on as well so it doesn't move around. A blanket or something on top is a good idea as well.
If your rabbit is litter trained and won't chew it, carpet can be good to use as well. It can be hard on their feet, so if they have sore hocks or thin fur on the feet you may not want to use carpet or will want to put a blanket or something on top.
Foam interlocking mats can also be used. Some rabbits will chew them though. Since they are 2ft wide (usually) you will need to cut them down to size.
Anything you use should be firm enough to provide support over the grids.
There are resting mats that breeders typically use in wire bottom cages. These are plastic and while they still have holes it is earier on the rabbits feet.
