Generally speaking, unless your dog is running the Iditarod (and is not a Siberian Husky) s/he does not necessarily need to "suit up". Most dog breeds have a coat that suits them in any type of weather and the addition of pet apparel may in fact cause them to overheat.
But, there are exceptions. Older dogs, greyhounds and small dogs (especially those with very short/smooth coat such as mini-dachshunds) do get chilly as the temperature drops, and you'll be able to tell because they tend to shiver. A good rule of thumb regarding whether or not your pooch needs to wear winter apparel is to simply check their comfort level when they are outdoors. Shivering is your best indicator. If your pooch doesn't seem at all fazed by the cold or snow, then s/he is already adequately "dressed" for the weather in her/his own coat.