I'm big on organization (my SO calls it OCD...) as well as meaningful URLs, so I have some ideas about this.
I'd say it all depends on the purpose and content of your site. If you've got a good amount of any one type of content -- be it blogs or whatever -- while also having other stuff, you might benefit from separating it out onto a subdomain. For example, if a site is entirely (or mostly) blogs/galleries, I might go with something like:
If I've got other content as well as blogs and such, I might go with a subdomain and keep my non-blog content on the main site.
http://www.example.org/ (main, non-blog stuff)
There aren't really any recommended "best practices" here -- it's all just personal preference. Basically, just look at your content (both current and planned) and figure out what makes the most sense.
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