Sourdough starters, like fermented sponges or fermented straight doughs are relatively safe due to their low pH (high acidity). The only cautionary word about starters, especially when developing a new one, is that you really don't know what you have growing in it. Typically, it will be some strain of lactobacillus, but there are no guarantees. While the acidity will protect a mature starter, it is entirely possible to develop aflatoxins from an unwanted fungus, or spore forming bacteria (clostridium) and this is where the potential for problems arises. While this would exist for any bread making process, not just a sourdough. In a sourdough, it is possible for clostridium to produce aflatoxins before the starter becomes sufficiently acidified to control it. Why would you want to taste a raw sour? The proof of the sour is in the flavor it imparts to the baked product.
Tom Lehmann/The Dough Doctor