The first two log cabin blocks I made ended up different sizes so I knew I had to do something different. What worked for me was to cut the strips at least a sixteenth of an inch wider than the pattern said, and after sewing on each round of strips, I trimmed them to the correct size.
Any time I can make quilting blocks slightly larger and then trim them to the correct size, I do it. Evidently my sewing machine is defective in some way and does not allow me to sew a perfect 1/4 inch seam :).
Here's the one and only log cabin I've made.