Apple Computer, Inc. has not done extensive testing on envelopes with cellophane or plastic windows, so we don't have recommendations one way or the other. However, there are some basic guidelines you should follow.
If you decide to use window envelopes, see if the paper manufacturer has made any recommendations on the amount of heat the cellophane or plastic can withstand. The various LaserWriter printers use different amounts of heat to transfer the toner to the paper depending on the model. The faster the printer (pages per minute), the more heat the LaserWriter fuser produces to adhere the toner to the paper.
There are also newer materials similar to tissue paper being used, which are less likely to cause any sort of problems in your LaserWriter printer.
Before using large quantities of envelopes, try one or two to make sure they work properly in your LaserWriter printer.