I think it’s a hundred times more important to know what something is than to know what the technical name for it is. (I can never be sure that I remember what litotes is. I think it’s using a double negative for purposes of dramatic understatement, eg “He was not unattractive.” Or zeugma, which I hope is making a verb do two or three things at once — as in “She drove away in a heartbroken condition and an elderly Ford truck.” “He lifted his spirits, his hat and her hopes.” I’m not even sure that I’ve spelled them right. And I’m not checking, because the point isn’t whether or not I’ve spelled them right, the point, if there is one, is that I know what the things being described are, and how to use them.)
How good does your knowledge of grammar have to be? Good enough to write something that’s obviously clear, understandable English. Good enough to know when you’re breaking the rules and to win an argument when you need to.
There are great musicians who can’t read music. There are great musicians who cannot tell you the names of chords. There are songwriters and composers who need other people to come in and tell them what they’ve been doing technically, but just because they do not know the names of things does not mean that they are not doing them, or that their work is inferior to that of a different composer who does know exactly what a diminished fifth is.
Me, I’m the kind of person who reads Fowler’s Modern English Usage for pleasure. I like English, like writing in it, like understanding how it’s put together. And mostly, as I said before, I like knowing what the rules are for when I break them, because I can quietly win arguments with editors or copy editors that way.