It won't ruin your fabric if you made a "length of fabric" cut rather than a "width of fabric" cut. If you did, you can still go back and cut on the width if you want to. You will only get 7 or so squares per strip, but just cut and extra strip (or 2) and you'll get the total number of squares you need. Just count the total you need and make sure you have it. Or, you can cut them all on the length of fabric. It makes no difference, as a square of fabric is a square of fabric and the direction will usually be lost as soon as it's cut.
It could make a difference (probably really small) in the amount of fabric used. For that reason, you might want to get everything cut and counted to make sure you have enough fabric. If you need a small amount more, the sellers are more likely to have it sooner than later.
bkay