Hm, I thought the point of sourdough was that you didn't use commercial yeast at all. I admit to boosting a slow culture with it for my last loaf, because I'd decided to bake too late to get the culture fully revived, but I felt like I was cheating. :-)
All that said...when I use yeast for bread baking, I have come to prefer Red Star. Whether my bread has improved because of some quality superior to Fleischman's yeast, or because I have been improving my bread techniques, I leave as an exercise for the reader.
