Aperture and cropping question?
"Do not constrain" just means Aperture doesn't maintain the aspect ratio of the original photo. The proportions you crop to may, or may not, match the proportions of the photo paper you are using.
It depends on the printer, but you usually have the option to either a) "fit" all your print into the photo paper, in which case you will see white unprinted bars on the print at top and bottom or on the sides of the print or b) to "fill" the photo paper, in which case you don't get white bars, but you do loose some of the print at either the sides or top and bottom.
You can always reset a crop in Aperture if you don't like it.