You can use Xcode's Organizer window to easily do actions such as adding a development device - saving you from logging into the developer's website and adding the device manually.
When using any of the Organizer's features that requires access to your developer's account for the first time, you'll be asked for your AppleID and password. Any subsequent use will be done silently without prompting you for those credentials.
What if you want to change the Apple ID you're using to a different account?
There's no setting or preference in which this is set. To do that, you need to go to the 'Keychain Access' application and filter the list to 'Passwords' (bottom left pane). Look for an entry called 'daw.apple.com' of kind 'Internet Password'. To verify this is what you need to delete, control-click it and choose 'Get Info'. In the window that pops up, you'll see the old Apple ID under Account.
Right click (control-click) the entry again and choose "Delete 'daw.apple.com'".
Quit and relaunch Xcode if it's open as it may have cached the old account information if it needed it. If you try to do something in the Organizer window that needs access to your developer account, you'll now be prompted for your new Apple ID account info.
