The LaserWriter Pro 600 and 630 are susceptible to label printing problems if the cassette tray is used. Chapter 5 of the user manual contains the following information:
Loading labels in the multipurpose tray
You can load one sheet of labels at a time into the multipurpose tray for manual printing.
Important: When you print labels, use the multipurpose tray. (This helps avoid paper jams and curling)"
The manual clearly states that labels should be fed through the multi-purpose tray, and that only one sheet should be placed on the tray at a time. The LaserWriter Pro 600/630 cassette feeding mechanism uses a single metal 'snubber' to prevent multiple sheets from being pulled from the cassette tray when the pickup roller rotates. Labels are made up of two parts, a backing sheet and the label material.
In some cases the 'snubber' can catch on the label sheet causing damage to the sheet or skewing it slightly so that it jams later in the winding paper path. The multi-purpose tray uses a friction retard pickup system which eliminates the need for a snubber, thereby improving the results when printing on labels.