[RCM78]

If you drop your helmet, the section that hits the ground will become compacted. The material used to absorbe energy in a helmet will not reexpand. Once it is crushed thats it. That means the one section of the helmet that has been crushed will no longer protect you in a crash. So if you do crash make sure you dont hit your head in that spot.

Multiple drops just make it worse.

Hanging a helmet on your mirror can cause two problems:
1. It will fall off and drop about 4-5 feet.
2. Everytime you hang the helmet on the mirror you slightly crush the material used to absorb energy in a crash. Slowly rendering the helmet useless.

Like Bob said, The proper place for your helmet is on the ground next to your bike. I put mine right behind the front wheel...