It depends. Were the stairs carpeted, wood or concrete/metal? If carpeted, I'd check it thoroughly and continue wearing it if it's ok. If wood, I'd think about it and if concrete/metal stairs then contact Giro and have it replaced under their crash replacement program, but shop around for prices on the helmet that you want as sometimes you can find it less expensive elsewhere.

A helmet falling down a flight of stairs can crack just as easily as falling 6 feet while being worn. It just depends on how it lands and where it's struck.

The other question you can ask yourself... how much is your head worth to risk wearing a possibly cracked/weakened helmet? If you have a spare head somewhere in your house, the risk it.