Oy vey. I don't know what George Carlin has to say on the subject of idiomatic constructions in English, but if we lived in a world where phrases could only be used in the most literal understandings of the literal definitions of their component words, idioms like "toe the line" wouldn't exist in the first place. The handy thing about it being an idiom is that it works even when the literal meaning, the context, and the history are forgotten. The difference between "toe" and "tow" is nil in terms of how it affects the meaning of the phrase, since the meaning of the phrase is not derived from adding together the meanings of the words.
