> Sounds convenient but IMHO somewhat beside "the point" of all these pontifications
It is until it isn't; most of the time when I deal with "normal people", they seem to expect getting sick from "bad" food almost immediately, and are confused why I haven't died of cancer already because I'm using artificial sweeteners with my tea. My rule of thumb applies primarily to that kind of situations.
Not denying the importance of long-term considerations, but given the amount of nonsense media publish (c.f. http://fivethirtyeight.com/features/you-cant-trust-what-you-...), I just ignore the topic altogether until some entity like WHO or FDA starts making noises.
Sounds convenient but IMHO somewhat beside "the point" of all these pontifications: your rule of thumb we as a species had already figured out for 100s of millenia+. That's why we don't eat grass and are told to be cautious around unknown wild berries or mushrooms. All these ponderations like OP OTOH are about "insidious, long-term-unnoticable-until-too-late health scares". An ongoing search for insights comparable in spirit to past "long-term damage" findings about lead or how some trichinosis parasites have an incubation period of ~30 years.