Relative paths or fully qualified paths?

I want to include a subscriber optin form in a sidebar widget. The subscriptin form calls some css and javascript. On my regular WP site I put a fully qualified pasth to the js and css located in the theme directory.

What do you suggest for WPMU if it will be called from a text widget? Should I place the css and js in each theme that will be using the form, or create one simple repository?