Mish, if the center you are working on is not a size that is easily divisible by the size of a particular block you want to make, you can always do it as partial blocks and partially plain fabric. I think that's why we see so many borders done that way. If you'll look at what I did on my round 2 on page 26 (I believe), you'll see that's what I did. That way you're not driving yourself crazy trying to make a 5" or 6" block work when it won't.
Email if I can help you further.