You can't always rely on a 1/4" foot to give you the appropriate seam allowance. Fabric and thread thicknesses vary and this will affect the size of the resultant patch. You often will need to move the needle or use a piece of tape to mark on the machine bed where the seam really needs to be.
Here's how to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
One other thing, if you can cut your strips parallel to the selvedge, they stretch less and I find I get a better more accurate block.