I see that girsempa 'put the icing on the cake' with,

you can always adjust your selection by choosing Select -> Transform Selection.

I tried that and it works exactly as you said. 

If you want to maintain the same aspect ratio, be sure to hold the
Shift key while dragging any of the corner handles

OK

All in all, this method will give me a way to visualize many possible ways to crop an image ... and, at the same time, select an image that will exactly match the paper size. 

BTW, this will also give me an easy way to create consistent square thumbnails for renderosity gallery uploads. 

Thanks for the help. 

--
Martin