my midwife has told me that it is safer obviously to make the bottles as you need them, however for night feeds etc this is just not always best.. especially with a starving, crying baby at early hours of the morning.. and waiting for the kettle to boil/cool down.
she went on to say that preparing a few bottles at a time fully, leaving to cool and then placing at the back of the fridge, leaving for a max of 24 hours, and just placing in a bottle warmer when you need them, is the best option (this is what i will do)
i have also heard of people making the water up in bottles, placing in the fridge and adding the formula as you need, however there is a lot of question about whether this is safe, as formula is meant to be heated to a certain temp in order to be safe, as formula milk is actually not sterile until hot.
I think for me the best thing is to make up a few bottles and place in the fridge, and warm up when needed. Always throw away after 24 hours and always keep at the back of the fridge, not in the door or anything, and always use sterile bottles x