Who's in charge of "tiers"?

This write up about the 2008 GOP field is getting some broad circulation--in part because it refers to the GOP pack as Lemons (as far as the GOP "right" is concerned). But it is an interesting piece because it is so "media-" and "registered voter-" centric. And I wonder why Rush Limbaugh (quoted at the beginning of the article) is so readily following suit. (here) Here's an excerpt:

"'Where do social conservatives go?' asked Dan Schnur, a California-based Republican consultant who worked for McCain in 2000 but is staying out of the 2008 contest. 'They've been the determining force in the nomination process for a generation and they've got no candidate in the top tier.'

Plenty of people are trying to prove Schnur wrong. McCain's advisers are betting that the disdain toward the Arizonan among many of the most vocal activists will not prove representative of primary voters. Running in a party that counts many Baptists, Romney may find a sympathetic audience for his story of conversion on social issues -- if he can convince people that he saw the light on the road to Damascus, not the White House."

My question is where's the home office or star chamber determining which "tier" a candidate belongs in? Uh, when did so-called Mainstream Media folks become reliable guides about who is or is not "credible," "electable" or even "likeable"? And since when is a poll of "registered voters" a reliable indicator for what primary voters are thinking?