Now that every married couple I know has a blog, the following seems timely.
When you're writing a surname, the same rules apply as they do to every apostrophe-s word out there. It's the 
Johnsons, not the
 Johnson's. If we're talking about the 
Johnsons' lawnmower, however, it looks like that — plural possessive. (I live in New York City, so I don't see a lot this on homes or mailboxes, but nationwide, I've seen a lot of "Welcome to the 
Hanson's" action going on.)
Last names that end in -s are understandably tricky and take an -es. 
The Joneses always seems to mess everybody up. It's either 
The Jones Family or 
The Joneses.
However, I admit there may be more than one school of thought on this topic. If yours differs from mine, please fire away!