...of course, uberculture said it much better than me. I personally don't care for fermentation control, so I use yeast, when I use it, with a wide tolerance range.
Seriously, though, with the same malt bill and hops, I could probably make 50 different beers by changing the microbes in the beer. But as far as where to start? As mentioned above, try to think about what you want from the yeast in terms of attenuation and esters/phenolics, and go from there.