If you're using it as litter, it won't be terribly absorbent or control the odor well, but it is ok to use. The only thing to watch for and be careful with, is if they start trying to eat too much of the straw, instead of eating their normal hay and pellets.
Straw is mostly indigestible fiber and doesn't contain much in the way of nutrients. A rabbit that is eating too much straw and not enough of their other nutrient rich foods, can start to lose weight and develop other health issues like nutrient deficiencies, fatty liver disease, or an impacted cecum. So do keep a close eye to make sure your rabbits continue eating hay and pellets normally.