I do both. If I am creating something that has elements that crosses the page I will do it together as a 12x24. Then if I want to print it I just save each 12x12 side individually. You just have to watch out for the middle because you can lose some of it in printing. I usually don't put anything I'm not willing to lose in the last 1/4 inch, or if you have the grid on, in the last square along the middle.
I also do pages individually if the pages are more blocked because it's easier for me to see the elements. I don't have a big monitor so I really have to zoom in sometimes and I can't get the over all picture with the zoom all the way in.
Hope that helps