I live in a larger house, being single any house is overkill but I went a bit beyond because of pricing and location. As such I have three bedrooms and two baths where the vents are kept closed. This does mean they seal completely so in winter the rooms can hit the 50s (F) on a below freezing week.
You don't need to live in a cold climate to want good insulation, it can save a bundle in hot climates too if your a fan of extreme AC; think 72F that some I know live by. I do the ceiling fan method, 78F and a nice big ceiling fan in every room.
Seriously, it is that simple.
/lives in a country where triple windows are standard for all new buildings intended for people.