Years ago, when I went fabric shopping with my Nanny, she would receive extra fabric maybe an inch or two with each yard as the sales lady unfolded the fabric from the bolt. ..I remember the thump thump the fabric board made on the cutting table as the cutter quickly measured the fabric against a yard stick and moving extra fabric after each yard.
Today, I now observe the meticulous movements of the cutter, as she ( I have only seen women cutters) unwrap the fabric ( no more thump thump) and positions the uneven edge against the metal yard stick ......going through multiple giyrations to get the precise yardage requested.
Because of this I usually ask for a 1/8 yard more just to square up.