I think a more interesting question would be: should food that contains grapefruit have a warning? Any food with high levels of phenylalanine says “phenylketonurics: contains phenylalanine”. Perhaps food containing substantial amounts of grapefruit should carry a similar warning.

This is complicated by the fact that other citrus fruits may have similar interactions and, at least according to Wikipedia, the full set of problematic citrus fruits is not well characterized.