It depends. If you are going to use other yahoo services like their IM and news, it is more convenient to log in there and use your one yahoo ID. The same goes for Gmail because it is integrated with the other Google services. LIkewise for microsoft's hotmail. If you have your email on one system and do your chatting, etc on another system you are likely to start getting emails in both places because people who chat with you will assume that they can use the same ID for emailing you.