Best surface for tires to sit on for extended periods?

Interesting. So it seems that, as most topics end up, its a battle of opinions. I could see where asphalt would potentially do harm do to the petroleum base. Currently my tires sit on wood as I didnt like them sitting on gravel and rocks. But I was thinking of taking some rubber barn mats and cutting them to park on. I guess I'm fine as I have them now. :)