I will say that we kept all our formula samples; we put them down with the earthquake supplies. I want my husband to be able to feed the baby if something should happen to me and, secondarily, if there is a major earthquake in our area, we will probably run into someone who needs that formula.
I've never actually opened a can of formula, however--all the samples we've ever received were stored until their expiration date, then pitched.
I would second the advice to just keep going in the first few weeks--newborns have tiny mouths (at least compared with my nipples) and there's some discomfort/pain at first, no matter how wide the latch or careful the mom.
The first rule of nursing newborns is to feed the baby--whether that means nursing, pumping, supplementing, or a combination. You cannot establish a good nursing relationship if you are terrified that the baby is going to dehydrate. If you don't feel confident, get help sooner rather than later. The second rule of nursing newborns is that they do not stay this young and helpless long; if you can keep pumping, you can establish your nursing relationship no matter what, whether that's at four days, four weeks, or four months. It took my first child many weeks of pumping and feeding, but she finally figured it out, and I nursed her for two years.