Lucy will shiver and request a coat or sweater in the winter. She started wanting one about 3 years ago, I just figure her old lady body can't regulate temperature as well as she used to.
Vi just wants whatever Lucy gets. She is my little heater and only really needs a coat if we are training outside in the dead of winter. She has a light shirt for in the house to wear so she doesn't throw a fit when Lucy gets a sweater.
Sune wears a sweater in the winter because she needs one. Most of the time the snow is higher than she is tall! Even though she has the double coat of her Pom half, she has no body fat and likes to be Chihuahua warm.
I also keep the house at 64F at night (since it gets warm enough with 2 people, 3 dogs, and 2 cats all piled in bed), 66F when I am home (since the dogs will all pile up and snuggle), and 68F during the day when I'm gone (I keep it a bit higher when the dogs are separated since they can't snuggle) to save on the gas bill. Hey, if I can save a $100 a month (actual savings, I tested it) by us all wearing a sweater and cuddling up, I sure will!