For those "tricky" people you could alternate your block sizes - just make sure they measure in multiples of 3" finished.
Example: Some made with 2-1/2" squares (6" finished), some made with 3-1/2" squared (9" finished), and some with 4-1/2" squares (12" finished)
Then set them with solid, embroidered, or applique blocks (also mixed sizes to fill in) and you would have a Sampler type.
I think this would be a neat way to use up some UFBs or create a "theme" quilt (butterflies, turtles, christmas, fall leaves, whatever strikes)