thanks Catdaddy - exactly!
One of the biggest users of exact size prints are the police forces - and many use a £15,000 software suite to render their files (and much more beside) - and its killer feature is an exact size module....
One way to get things exact is to make a print of something with a given size - then measure its size as the print. SO if you have something that measures 200mm on a 10x8 print and in the print its 208mm you'll have to resize it by 200/208 - so it ends up 192mm in the file....
This magnification causes issues on some album designs - if there are multiple images on the sheet that have to line up with apertures in the overlay.... - and also if you are adding text - as if you put it too close to the edge it will get cut off.....
If you are printing with an inkjet printer there is often little or no magnification issues...