If I add the Social Bookmark creator plugin to my WP, do I need to register an account with each of the sites I put on there. Do I need to belong to Digg, StumbleUpon, etc.?
You will not need an account with the different Social Media sites. The readers that bookmark your site will need an account with them though.
I would recommend adding Sphinn to your site. Google seems to like Sphinn. I have seen some of the pages I have bookmarked show up in Google on the same day.
Socialize-it/onlywire.com have features that let you bookmark to multiple sites with one click.
For Onlywire, I must add... that you need to setup an account at each social bookmarking site yourself.
But the plugin you're talking about is for the end-user. But it is handy when you use those buttons yourself, that way you have at least 1 Digg or Stumble on your post to start with.
Try addthis.com or ekstreme.com's socializer. And yes, you have to register in every social bookmarking site.
I use the social bookmarking drop down. I've tried several others, but the drop down is unobtrusive and highly configurable.