Highly recommended and advised. We lived for 15 years with just the steel entrance doors - three of them. Never gave it much thought - they were solid, well insulated, and for the most part tight. Finally decided this fall, after doing some weatherstrip and trim maintenance, that adding storm doors could do nothing but good - thought about it for years but always put it off. One of our doors opens into the living room on NE side from a verandah. Not used for entry during the winter, but there were always complaints of chillyness from some when sitting near it in the evenings. Another one enters our kitchen from a deck on the SW side - it gets used the most, as does the room it enters into. The third on a deck on SE side into a central hallway with not a lot of traffic. I did the NW & SW ones first, aiming to reduce cold air entry into the living space. Then did the SE one almost as an afterthought, since it entered into an area not really habitated much. But here I sit on a sunny morning, it is -15c outside, the steel door is open on the SE hallway side, sun is blasting in through the new storm door reaching all the way into my office 25 feet away on the other side of the house, the entire first floor is up to temp and will be all day with no furnace help. I severely underestimated the impact the storm doors would have on not just keeping cold air out, but increasing my solar gain by a ton on days like this. I am amazed what that one new 3ft x 80in. piece of glass on the SE side of my house is doing for my heating situation. Cross ventilation in the summer will be an added bonus to come. Seems common sense now, but took me 15 years to get around to it.