I had lots of shirts and was wondering the same thing. I finally cut them all into 5" charm squares, reserving the left front (the side that normally has the pocket) which I kept whole. This worked for me. I have mixed them with other fabrics, but usually just use the shirts. If I decide I want some sashing, I go & buy some shirts of the right color for this specific use. Here's some of the ones I've made:
http://www.quiltingboard.com/picture...s-t153884.html
http://www.quiltingboard.com/picture...t-t209062.html