There's different ways to cut binding. Cutting with the selvedge is one way to have the least stretching of the binding. I frequently cut my borders that way, but never the binding. Stretchy binding isn't a problem for me...I usually cut mine on the bias which is the stretchiest of the three... with the selvedge, across the grain or bias. She sounds like she mostly sews clothes. When making clothes, you don't want a stretchy cut, almost everything is cut with the straight of grain which is running with the selvedge. Sounds like you have figured out how to calculate your own yardage for the binding. That's best, that way you will cut it your way.