Logging out of your account when you're done using a computer other than your own is just good security. If you use multiple accounts and want to simplify the process, Super Logout can log you out of over 30 major services at once.
As soon as you visit the site below, it will automatically get to work logging you out of a few dozen major services including Amazon, Google, Netflix, Steam, Tumblr, and YouTube. Once you're logged out, the site will display a green "OK" next to each service. The list isn't comprehensive (notably, Facebook is absent), but it's a quick way to log out of many sites at once.