Crop marks are useful when printing on plain paper and cutting the designs manually.
To print crop marks, check 'Crop Marks' in the 'Print' dialog box. Note that the 'Print' dialog will remember this setting until you turn it off again. The crop mark will be printed at the outside of each label nearest the edge of the paper extending to the edge of the paper. The crop marks printed in this fashion to help manual cutting. In such cases, crop marks will obviously not be printed.