Greg Noonan once told me to ferment my lager starters at fermentation temp. But, I have to admit, I have tried it both ways and don't seem to notice a difference. You are growing yeast, not making beer. Like Gordon said - it ain't gonna morph into ale yeast in one gen. Theoretically you are selecting the warmer fermenting yeast. But, OTOH, its not like the colder fermenting strains are sitting there doing nothing the whole time.