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.
To add to the above, it can be hard for a BFing mom b/c you can not measure what the baby is eating. I have seen too many moms start out great but doubt themselves b/c there was nothing to measure and start supplementing and eventually giving up. You need to trust your body and it's ability to feed your baby. I would seek help way before I would even think about supplementing a newborn. (even with BM) You need them to be sucking to produce and I would keep the baby latched on all day long if you think they may not be getting enough.
Also my DD was a screamer she screamer all-day-long with all the BFing she could handle and me wearing her skin to skin 12hrs a day if not more! Some babies are really high needs and don't doubt your parenting if you have a baby like this. The first year of her life was the most challenging of mine but I met her needs and never took the easy route (not AP) I always stuck to AP even with all the hardship I was going through and around 9 m/o it all started to change and at a year she was absolutely amazing! Now she is 19 m/o and so secure and happy and loving I couldn't ask for more. I know the way I parented her met her needs and let her be who she is. She truly trusts me and all I get are compliments about how well behaved she is and how secure she is. She will literally be at the park running up the big kids playground to the biggest slide and there will be something dangerous to one side of her (like a drop with no rail or something) and all I have to do is say "uhuh danger" and she will immediately turn around and just go down the slide. That happened yesterday actually and several parents came up to me in awe about how secure she was to be able to run through the park and really listen to me if need be. Anyway my point is don't panic and think you are doing something wrong if you have this kind of baby. I had a few moments of "OH NO what if she isn't getting enough milk" and on top of that my pedi stunk and was pushing formula from our first visit. I never gave her any though. I kept in touch with my MW and she always assured me she was doing great! When you have a "harder" baby it is easy to blame yourself that there MUST be something you are not doing right, but it's not always true. So trust yourself, trust your body, and trust your instincts. Don't let anyone else try to convince you that you are doing anything wrong b/c you will get a lot of unwanted advice!
Young born-again mama and loving wife to DH and SAHP to two crazy girls we and believe !