hyphenation - When to hyphenate open-form compound nouns

I am taking an editing course, and the instructor said that the following phrase must be hyphenated and that I was wrong in not hyphenating it:

row-house kitchen

I did not hyphenate row house in this phrase, since I thought that it was clear that the kitchen belonged to the row house. To me, it seems a bit silly to presume that the reader would be confused here. (However, I have lived in a row house in D.C., so perhaps I am biased.)

Must row house be hyphenated in this instance? If so, then must, for example, free[-]trade agreement or post[-]office box be hyphenated? This seems excessive. I searched several periodicals, and these phrases appear without hyphens more often than with hyphens.

I see that there is guidance here and here, and I understand that one should employ a hyphen to improve readability where necessary, but I am having trouble understanding when and when not to hyphenate an open-form compound noun. Is this more of a style preference? Thank you very much for your help.