You should always be organizing your layers into a group when using Photoshop.
I was working on an assignment for my Digital Design class where I had to build a series of mock up web pages. The requested size was to fit the standard 1024 pixel screen width, but when I was working on it I had it at 960 pixel width.
When I realized that I had it at the wrong width I had already finished a couple of pages. If I had not grouped all of my layers, then moving everything to fit the 1024 pixel width would have been a pain.