Best is to make up fresh. Second best is to make up with hot water and keep cool in fridge.
You can make up fresh with thermos-hot water, if you like, or you can make it 50/50 with hot water and cool boiled water, just make sure the hot hits the powder first, and you have to measure very carefully. This is in effect what the Perfect Prep machine does.
It's important not to make it up with cold water because the powder cannot be sterile, if there are any germs in the powder, cold water won't kill them. In the quite recent past, the message was about not making bottles up in advance which is why a lot of people made up as and when but with cold water. Now the focus is changing, because they have realised that it's silly to tell people to only make them up as and when, it's not practical and nobody sticks to it. So now the hot water part is emphasised, but there are a lot of conflicting messages.
Basically you're trying to minimise the amount of bacteria present in the milk. Babies' immune systems can easily handle a small amount of bacteria but if it multiplies to higher levels it becomes dangerous and can cause stomach upsets or, rarely, worse.
So we can do this in several ways:
- Keep the environment as sterile as possible. Wash bottles and teats well, sterilise, don't cross contaminate. This ensures that no new bacteria is introduced.
- Use hot water to make up feeds. This kills, or at least reduces, any bacteria which is present in the powder. You can also use ready-to-feed formula which is sterilised and can be kept at room temperature (after opening treat as made-up powder formula).
- Keep milk out of the "danger zone" which is between 4-60C. This is the temperature at which bacteria can thrive and grow most easily. So cool feeds quickly rather than letting them meander down from 70-100 to a drinkable temperature, and keep feeds made in advance at the back of the fridge until shortly before you want to feed them.
- Don't keep made-up feeds for too long. I believe it's 2 hours at room temperature, or 24 hours in the fridge for formula. Expressed breastmilk has different times. Because the baby's mouth can introduce bacteria, you should discard a partially-drunk bottle after an hour, too.
If you know why a guideline exists, you can see more easily whether it makes sense to stick to or break it. These are the safety concerns that I know of relating to bacteria - there are other safety concerns such as milk being too hot, sodium content in the water, etc.