Not necessarily - you can have a standalone blog (e.g. https://github.com/swanson/swanson.github.com). GH Pages does allow you to easily setup a project page per repository, but it is not required.
Thanks for writing this, swanson. I've written on and off over the last few years but lately I've wanted to write, started to write, and then lost the nerve and never posted. Hearing "you don't have to be the next Joel On Software" helps.