Everything is toxic to an extent. The key consideration is concentration of the material in question. You will die in a full Oxygen environment, but need Oxygen to survive. Likewise, you need water to live, but every year people drown in water.
I went back to an early Brew Strong episode where John and Jamil interviewed Jon Herskovits of Five Star Chemicals about sanitation. Jon mentions that the 'breakdown products' of Star-San may be utilized by yeasts. He notes that this is in response to heavily diluted amounts of Star-San (as in residual foam). [citation: Brew Strong episode 'Sanitation with 5-star' airing 1/5/09]. In reality, they would be in such a dilute concentration that I don't know how one could definitively state one way or the other.
In usable form, diluted according to instructions, Star-San is an accomplished sanitizing agent, which is to say that it destroys or cripples bacteria, yeasts, and other organisms to prevent and impede them from multiplying.
I use Star-San to sanitize my equipment just before use. Depending upon how I left it, I have sometimes treated my flasks with Star-San and left the foam in the bottom as I pour in starter wort. While I have not studied it in depth and do not have a clean study to resort to, my cell counts on those starters which have been made on some residual Star-San are well within what I would expect for cell counts and not differentiated from those starters where the Star-San had long since dried before use. Given the wide amount of error in my cell counts, this does not amount to much except for anecdotal evidence (very weak).