Another question answered: When you cut a strip of fabric, and it's not cut quite straight, it will make a 'V' in the strip.
To keep this from happening: Fold your fabric so that the selvedge edges meet each other. If they don't quite match up, move them a little up or down, until it is laying very straight and flat.
Fold the folded edge over, so that it touches the selvedge edge and lays flat.
Now, take the largest ruler you own and lay on the new fold. Make sure that the markings on your ruler are even. If you do not own a large ruler (I use my 12 1/2" when the fabric will allow for it), you can lay another ruler under the one that you are cutting with, and line up the numbers on both rulers and hold your cutting ruler in that position. (This is what I am showing in the picture.)
Then, cut.
After several cuts, you need to check to make sure that the edge is still straight. We all move them slightly and that can change the cut again.