Where should you use StreetPass? Anywhere! Keep your Nintendo 3DS in a pocket or bag, go about your business, and StreetPass will do the rest. If another person also has wireless connectivity and StreetPass enabled, then you'll exchange tags when you pass by each other—all while your systems are in Sleep Mode.
Be sure to turn on the StreetPass feature in each game, so you can take full advantage of StreetPass tags and information.