Baby feedings

Jus wondering, should we prepare the milk for baby in advance before he/she wake up? Or store the milk (put in warmer) for more than 1 hr ? My MIL always prepare the milk in advance before my baby wake up, sometimes my baby sleep longer than expected then she just put in the warmer untill the baby wake up. I feel this is inappropriate, as the milk is not fresh anymore... she said to prevent baby from crying and no need rushing to prepare when needed. She prepare the formula using 100° boiling water and rest until it cold, she said cannot mix with room temperature water.But according to the instructions on the milk can, it says cannot use 100°c boiling water to prepare the milk.. Mummies out there, any advise? First time mum here, no experience..