The problem with any sort of shared login system is that the system is only as strong as its weakest link. A security breach at any one of the sites you use will provide access to your account at all the others. It's much better imho to use different user names and passwords for everything and store them in some encrypted form on a floppy in case you forget (so you don't have to use the secondary verification systems, which are generally weaker).

If you don't care about security, just use the same user name (or as close to same as you can get) and password everywhere.