I don't use the index card anymore because although the marked lines are 1/4 inch apart on the card that doesn't apply to my pressed seam. You loose a little in the pressing. This isn't really a big deal if you are using the same foot, etc on a quilt you make yourself. I gets more critical if you are trying to do something with a lot points and matched seams.
Getting a good 1/4 inch seam can be a big deal if you are swapping with someone and your pattern says it will make a 12.5 inch block and your only comes out to 12 3/8. That is just frustrating.