Through the Finder, your username and password constitute approval to initiate screen sharing, so yes, you can do it to a computer with no one near it. Keep in mind to do it over the internet, you need to know that computer's public IP address (if it has one of it's own) and have the proper ports open on your router(s), or use MobileMe and Back to my Mac (usually works fine, though I have had problems on occasion).
jW