If you have a self-hosted blog, using free software from WordPress.ORG, then you can do whatever you like.
But if you want to map your blog to an existing domain, then you are still subject to the TOS here.
Same Rules Apply – The Domain Mapping Upgrade does not enable the permission to use advertising, any kind of prohibited code, or upload additional themes or plugins. With the upgrade, your blog will still be hosted here at WordPress.com, which means that you will not have FTP access to your files and you will still be required abide by our Terms of Service.
If you have a question about whether or not an action on your part is against the Terms of Service, then you must ask staff. We volunteers in these forums have no authority to make such a determination.