You don't have to pick between fabric OR ruler with those iphone app coupons!
As long as they have different numbers on the coupon (scan numbers) you can use more than one at a time - one coupon per item, though, as far as I know.
Also, the app coupons can be used over and over as long as they're still valid. I've used the one that's good all of October 4 times this month. So, if you use a coupon for a cut of fabric today, and it's the only one you have, go back tomorrow and use it again on the ruler you want, if it hasn't expired.