I used to believe that the yeast/bacteria present in the flour would eventually become the dominant life forms; it seemed intuitive. However, after reading a bit more on the topic, I no longer believe that. From what I've recently read in various places, is that given proper hygiene and a proper hospitable home, the established yeast will remain entirely dominant, for many of the reasons stated in the last post. They are already active and breading.
Cross contamination is probably a much bigger concern, when propagating multiple strains. Of course if you let your culture go dormant by not caring for it, then you're getting risky, but even then the risks seem very minimal at best given the ratios.
A previous poster mentioned airborne yeast, which is a common misconception. The yeast and bacteria are present in the flour, not in the air. Lots of nasties in the air, but nothing you're likely interested in propagating.