Does that sound correct? And if so, is it a good practice to not specify the template group?
As with so many things, it depends on the site in question. As was mentioned earlier, you don’t have to use template groups at all. In theory, you could use just one massive template to drive an entire site. Of course, that’s rarely ideal because the template would likely be far to complicated to manage and maintain efficiently.
However, you don’t want to go overboard and have a template group for every single page on your site.
My personal preference is if I have a batch of templates that are for a common purpose, or make up a single section, than I create a template group for them. For example, if I have a site with a blog, chances are there will be several templates that will be used: one for archives, one for single entry pages, one for comment previews, and so on. And so I would create a template group to contain all of them, just to keep things neat and tidy.
But like I said, that’s my personal preference, and sometimes, depending on the site I’m building, it doesn’t make sense. The nice thing about EE is that it doesn’t force you to structure your site in any one way. Of course, that’s a bit of a double-edged sword, because it does require a bit more forethought to ensure that the method you’re using is the most efficient one.