Choose a focal-point photo. Treat other photos as "accessories" that help you to tell the rest of the story. As we read from left to right, the top-left hand corner is an excellent point for your focal-point photo when you have several photos you want to use on your layout. You don't have to mat every single photo on your scrapbook layout, unless you want it to stand out from the background. Always mat your focal-point photo though.
This tip was taken from: http://www.everything-about-scrapbooking.com/scrapbooking-tips.html