Who wants snow tracked through the house by potential buyers? You want them to see it, but you don't want the mess. Put down a bright and cheerfully colored doormat. This serves two purposes: keeps the house clean, and says "welcome" at the same time.
You could also provide umbrellas by the back door. Those wishing to see the yard can protect themselves from the elements. They'll appreciate it. The secondary benefit to you? The buyers perceive your attention to detail as indicative as to how well you maintain your home.
Remember to keep the heat on, too. You want to convey warmth in every sense of the word. If the buyers are cold, they won't want to linger in the house. Make sure they feel at home. After all, your goal is to make this THEIR home, right?