If you do minor changes in the default template, you do not need to create an extension with the template. Just keep in mind that updates might effect your template files. We try to keep minimum effect on the templates in upgrades, but there is always a chance that there will be a fix in the template files.
If you intend to create completely custom template, we suggest you create an extension. This is a developers job. If you are not developer, this is not for you.