How necessary is formula sterilizing?

I have a 3.5-week-old newborn who is mostly formula-fed. I was in the hospital longer than she was, and my sister-in-law took care of her for the first week. As an inexperienced mom, I just followed her steps. She was making formula with room temperature boiled water, and the baby eats great. Now that I’m educating myself more, I realize that an opened can of formula could be contaminated and should be mixed with hot water, then chilled before serving. We haven’t been doing that. How dangerous is it really? Do you always do that? Is it only necessary for newborns or always while formula feeding? I do sterilize the bottles and always wash my hands of course before preparing the formula.