Yes, recs are to use boiled water over 70deg due to the fact that formula is not sterile and can contain various contaminates (anything from bacteria to, in some cases live (or dead) insects). When I was mixed feeding DS I always just used the water straight out of the kettle and then cooled it down before feeding to him (or filled a metal thermos flask with boiling water if I was going out so it hadn't cooled too much before making up).