I think the really important thing is to use boiling - NOT just boiled - water to make the bottles up in the first place, and then to keep at the back and bottom of the fridge no longer than 14 hours. Because the powder is not sterile, and needs hot water to kill bugs.
The biggest problem is apparently people who don't realise that milk powder isn't sterile - at all - and in tests has been shown to be full of bugs. You need to kill the bugs in the powder with the boiling water, whereas many people think you need to boil the water so it has no bugs in, and then it's safe to use at room temp with the powder. I've sat in cafes with friends making bottles up that way; scarily intelligent women who didn't know the risk. (I feel shit because I didn't say anything, but as they were friends who were sad enough about not being able to breastfeed, and with my second I could, I wasn't about to be the arse who rubbed their nose in it.)
The best advice is to make every bottle up freshly, but I think that's because a lot of sleep deprived mothers use bottles over a day old. If you're disciplined enough to bin any left in the fridge at set times a day, and you are scrupulous about sterilising everything prior to making a bottle up, then it's not likely to cause any harm.
Personally I found the sleep dep so awful I couldn't even remember how many scoops, so with my first, when finally (thank God) moving to formula from expressed milk, I used premade. It cost a fortune but I had severe PND after a horrific experience trying to breastfeed a baby with undiagnosed tongue-tie, so the peace of mind was worth it.
And as someone who fed my first exclusively expressed milk for his first few months (and should not have done, for his sake and for my own), and who exclusively breastfed my second and only stopped on her second birthday, breast is NOT ALWAYS BEST. The mother matters as well. PND affects babies too. Breast is definitely the right feeding choice most of the time, but it can be a bloody awful one some of the time. Telling women who have chosen to feed formula, for whatever reason, that they are missing a trick with breastmilk is very likely to be rubbing their noses in something that hurts.