In the Midwest, I think it's usual to say "excuse me" if you need to address a stranger for any reason, especially if the stranger is paying attention to something else when you approach them.

For practical reasons, you've got to say or do something to get their attention before you begin stating your request. Otherwise, you might be in the middle of your sentence by the time they realize you're talking to them, and they won't have been listening, so you'll have to repeat.

You don't need to say "excuse me" if the person is obviously waiting for you to begin a transaction—say, the ticket seller at a theater.

The whole topic of politeness/etiquette deserves its own thread. I think I'll start one. Give me a few minutes to compose my first message, then look for it. (Please!)