I've got most of mine only because I'm too lazy to clean the old ones out. Just toss the new ones in the envelope and off I go. But yeah, I would say your last couple are the important ones. I do like the idea of marking the mileage on the receipt though, will have to start doing that.
And if you get did bad fuel, then so did a lot of other folks if you buy from a high volume pump. Which makes it all that much easier to figure out where it came from and prove that they owe you for crapping up your rig.