I would say the only problem is when they do not take the whole bottle. You know, for me pumping is hard work and every drop of breast milk is valueable, and... When DS is not taking te whole prepared portion and it was already warmed/mixed with formula, I would have to throw it away. So... If your LO are draining the bottles, why not. However in my case I never know exactly how much will DS feed so I would firstly give him BM and than topup with formula. If formula is not drunk, I wouldn't mind throwing the rest of it, ykwim?