Good advice; I always crop my images to the intended format (4x6, 8x10, etc.) before having them printed. Otherwise you have no control over how they will be cropped by the printing machinery and/or technician.
If you want to keep the full image, here is something you can do if you have Photoshop or Elements:
Select the crop tool and set it to the desired format (8x10, for example). Next, use the tool to draw a crop box on the image -- doesn't matter where or how large. Now, drag a corner of the crop box outside the image. Then enlarge the crop box to take in the entire image and overlap into the space outside the image. Finally, click the OK/Accept check mark. You have now "cropped" your image to include extra "canvas" outside the original image. When you have the image printed you should get your entire picture plus the extra while space, which can be trimmed off if desired.
Note that you can't (at least in Elements) draw the initial crop box to extend outside the image. You have to draw it within the image borders first, then drag it outside.
This can be a handy trick if you want to add a caption to an image or perhaps "dress up" an image with a fancy border.