As the discussion on sourdough organisms, flavor, and sourness goes on, I would suggest reading two long pieces which might help us all. And, contrary to what several regulars on this forum note, the information supports the contention that lactic acid bacteria enjoy higher temperatures, while yeast are most active in room temperature or lower environments. Of course, only experience can get us into the balance of flavor and sour we enjoy in a loaf.
From 2003: http://forums.egullet.org/topic/27634-sourdough-bread/#entry1769486
And from 2009: http://www.thefreshloaf.com/node/10375/lactic-acid-fermentation-sourdough