One of the guidelines for preparing your Photoshop or Gimp PSD web site design for conversion here at psd2css Online is called The Final Crop. The instructions read - "...it's a good idea to crop the image to exactly the size it is already". That has caused some confusion for some and since it doesn't sound like it makes any sense, some people don't do it. But sometimes it's an extremely good idea. Here is a quick video explaining what The Final Crop is and why it's a good idea.
Actually, the biggest problem that 'The Final Crop' can solve is when you have a layer that is supposed to be inside another layer but in fact it doesn't. For example anything that is supposed to be inside one big _center background layer, and in fact is NOT inside the _center layer because it sticks outside of the visible area. The Final Crop will fix those problems too.