Reach out to the middle: 49%
Pick VP to unite the GOP: 28%
John Damnedifhedoesnt: 15%
Rubio, Fortuno, or Martinez: 8%
Though a majority of voters thought that Romney should appeal to moderates, many of the commenters thought that Romney would try to fire up the conservatives that he personally has little appeal toward.
Cindy: I think the idea of Romney rallying the base in any way is a false hope, just as it turned out to be with McCain.
He has to try and swipe some from the center to make up the loss of votes from low enthusiasm in his base and to steal a few from Obama.
Oldskool: Four years later and Rs still need a hail mary. Someday, it may dawn on them that good policies make good politics.
However, conservative and moderate might not be as important as other contrasts between presidential candidate and running mate:
Michael L. I think center vs. right is a less important choice then "young, good-looking candidate who isn't ready to be President" (e.g. Rubio, McDonnell) vs. "boring grownup who won't embarrass you" (e.g. Rob Portman, Mitch Daniels). Just to add to that: there are grownups out there who could unite the party too- Kyl of Arizona for example.
Finally, several other commenters suggested randomly picking a running mate was probably just as effective as any other more rational method:
Patty D. Darts, with the actual people considered.