CodeCanyon previews - free or premium Wordpress themes?

For the items you sell on CC (especially for the Wordpress plugins), what you use on the preview website: a free or a premium wordpress theme?

I mean which one is allowed?

Both is allowed, does not matter.

I use default twenty eleven theme. From now on I’ll use TwentyTwelve

Thanks. I was confused by all these stories about wordpress banning codecanyon members from WordCamp…I wanted to make sure, but still…I am not so sure if themes from wordpress.org are free for both personal & commercial use. I just decided I will ask for permission to theme’s developer, maybe that’s the safest way to do that.

You can do whatever you want with themes from wp .org You can even sell them.