As picayune as that sounds, it is as important to "touch that base" as it is to make sure the envelope is addressed correctly. And make no mistake, there is indeed a definitive way to address envelopes, based on whether the person is single, married, a child or even widowed. Paying attention to how the envelope is addressed, making sure the correct method is used is a huge part of sending out cards and letters anytime, let alone over the holiday season. There are great style guides available either from your favorite bookseller or online that goes over all of the different addressing styles. That way you can know for sure you're following that bit of etiquette.
Other parts of the card or letter are just as formalized as the address. Everything from the salutation through the closing has an entire section of etiquette style devoted to it. While the main content of the letter can be whatever you'd like, there is still a style that has to be adhered to if you want to stick with proper protocol. While most of us have dispensed with much of this over the years, if you are going to be corresponding with someone who does have some level of royalty or in a position of power, sticking with the "right way" is generally the best way.
While using the postal service to deliver the card or letter is a necessary evil, according to the style guides, having the envelope properly address, the postage in the exact proper location, and having the return address properly affixed are all important as well. And having that envelope seal that was mentioned earlier is still a necessity. Heralding back to the days when the King would seal his correspondences with a wax seal to ensure privacy, envelope seals and stickers have come to represent that nod to propriety.
Granted, following all these rules of etiquette certainly complicates the mailing of a simple Christmas card, but by the time you put all the effort into posting a card or letter following proper protocol, it isn't a "simple" Christmas card any longer.