Most photographers I know are perfectly fine with you asking for them to re-crop a photo for you, especially if you say something like, you are asking them so they can retain the integrity of quality and their watermark.
Most image sizes for printing fall under the 2x3 ratio, but people often order 8x10s which do not fit that ratio, so the image will have to be re-cropped for an 8x10 to make it look right if they were saved in 2x3 aspect ratio. If you have the option to order 8x12, that works fine with the ratio and the image can usually be saved as is.