I'm not sure who starts it, but they're very clearly (and openly) rolling it out based on not only logical locations, but willingness of cities. Some places are just hard to get into, like Seattle[0].
From an email I got from the Google Fiber team last May:
"We’ve been impressed by how enthusiastic the City of Mountain View has been as we’ve worked with them over the past few months — so let’s take a moment to thank them for everything they’ve done so far."
The City is also working on the physical infrastructure Google needs:
I can't speak for other cities in the area, but for Mountain View, the belief that Google Fiber expansion is being held up by government inaction or unwillingness, is unwarranted.
(The one exception is San Francisco, which is a mess whenever anyone wants to build anything anywhere in the City, but SF's an outlier in that regard)
[0]: http://crosscut.com/2014/03/04/business/118993/google-fiber-...