Hay and straw will hold in moisture making a wet gooey mess if peed on. If not cleaned out it will mold within a few days and then if the rabbit eats it they can get very ill or die. We lost a guinea pig that way. Shavings (no cedar) or pellets (pine or paper) is the best choice for a solid floor and then if you really want you can put some straw over that but really it will only counteract the absorbency of the other bedding. If I do straw in an animal pen I lay down pine pellets, then pine shavings, and then the straw so that there is a very absorbent thick layer underneath the straw in order to collect all the moisture.
Hay should be available all the time in a hay rack or cleaned out frequently so it's always clean and fresh with no urine and possible mold growing on it.