Either option is acceptable. Handwriting the addresses follows traditional etiquette and adds a nice personal touch if you have good penmanship. You can also hire a professional calligrapher for beautiful, impressive results. However, there is nothing stopping you from printing the envelopes with a crisp, clear font. This method can save you a lot of time.

Ultimately, the decision is yours. However, make sure to avoid messy handwriting or printed address labels. If someone other than you handwrites the addresses, supervise them to make sure it is done properly.