So...if I'm an individual mailing Christmas cards, it wouldn't make any sense for me to use a 9-digit zip code?
The 9-digit zip code only makes sense if you're a business doing mass mailings?
Well - if you are sure you know the proper 9 digits, it may make it less likely that the cards get mis-sorted, but there's really no reason to use it for Christmas cards.
If you mess up the last 4 digits, it is more likely the cards will be late.
I wouldn't worry about using zip+4 if I were you... No really big advantage.
OBTW, here's a link if you are interested, but you will have to type in all the addresses to get the last 4 digits:
http://zip4.usps.com/zip4/welcome.jsp