There is a lot of mixed information about this!
I know Knickers Twickers did a lot of research about the issue, I now do what she advised.
I boil the kettle with frech water, give it a few minutes to cool down and then make up bottle. The water is hot to kill any bacteria in the milk. Then I cool it to right temp by putting it in a bowl of cold water. You have to use the water in the kettle within 20?mins as then it goes to cool to kill bacteria.
If I am making in advance. I do the same, cool it rapidly made up by a bowl of really cold water and then straight in the back of the fridge. I heat in microwave when needed.
I only make them up 12 hours at a time.
If leaving boiled water on side, its not hot enough when mixing with powder to kill bacteria.
But I know many do it this way and have never had problems! So it is down to personal choice!!