After my mother trims sheets she gets donated from a motel to be used in her LWR charity quilts (60 x 80), she cuts 10" squares for me from the remnants (bless her heart). I do strip blocks or sew and flip on those squares as a foundation. After piecing, I trim to 9.5" and clip the corner off the foundation for less bulk at seam intersections.
One advantage of using old sheets is that no pre-shrinking is needed.
Dayle