You can do whatever you like. Wherever the tracking code is will be tracked. I typically execute 3 tags per page. One for the site, another for the class of site (blogs, social media, etc.), and one tag that’s executed on all my sites.
The all subdomains or one subdomain affects how the cookie is stored (if you understand cookies, that will make sense). I typically say use subdomains, but even so when I track multiple sites with one cookie the session information work across all domains (somehow). The result is that the “all” cookie has longer “time on site” numbers than any particular site.