You definitely need to put your non-standard ratio images on a standard size canvas. This is because the default for most (all?) labs is to crop images to fit the chosen paper size. ie if you submit a 4:3 image from a digital compact and have it printed on 6x4 paper the image will be cropped on both sides on the long axis. That is because most images fit the paper fairly well, so it is better to crop them slightly than to get unsightly little white edges to the print.