Mockup strategy for Responsive Web Design

When making mockups for Responsive Web Design projects, should I separate them by Page or by Screen Size?

What are the pros and cons of the following:

• Page based: All the home page mockups in all different screen sizes


• Screen-size based: All the pages for width <= 960px

I'm using Balsamiq Mockups to make mockups and I prefer to store all related mockups in a file like homepage, login or 960px, 768px.