The convention is that if the whole family is invited, you can just put "Smith Family." It’ll save you some space; of course, if you’d like to personalize it, you can put all their names.
I’m freakishly into the spreadsheets, so I had a LOT of columns. Also, I wanted to use the same sheet for both the StDs and the invites. So, my name-related columns were:
No.: I numbered everyone from 1 on so I’d have a key list (pencil the #s on the back of the RSVP cards in case you can’t read names or people fill ’em in weird)
Category: Bride’s family, Groom’s family, Couple’s friends, Groom’s Parents’ Friends, etc.
Formal Name: Mr. & Mrs. Steve & Sue Smith (I like to put both first names, altho’ using just the guy’s is common)
Casual: Steve & Sue
First Name: Mr. & Mrs. Steve & Sue
Last Name: Smith (the reason I broke first and last like that is b/c I did my invites on the computer and did a fancy, large font and a swirly embellishment for the last names, so I needed them separate from the rest of the name which was set higher up the envelope in a block print)
Inner Envelope: Mr. & Mrs. Smith
As a side note, for those couples who live together or are married and have different last names, I did the first name field "Mr. Steve & Ms. Sue" and the last name "Smith & Jones"
I used the whole spreadsheet to track everything from sending the StDs, RSVP responses, whether they were taking hotel rooms on block, if they needed a ride in the shuttle, and what they gave as a gift (so I could use the list again for thank you notes).