Since quilt police are just a myth...I cut my sashing and border strips in which ever way will give me the length without seams, if I have enough fabric. If it is a busy enough print than it probably won't be noticed if there is a seam.
I can never remember which way (other than bias) is stretchy anyway! My excuse to daughters has always been "If it isn't life threatening, I don't have to remember it." It's worked for me for 30 years now (that's the age of the oldest)!
kathyd