Are the rules of grammar and punctuation very important? My answer is NO. What is important is COMMUNICATION. I want to find out what’s in the writers’ mind. The rules of writing and punctuation help the author write a clear sentence. I find out what’s in the author’s mind. Communication has happened.
So grammar and punctuation are only important in that they aid communication.
Further, better than looking up the rules, read your work out loud. For sure, you will hear the punctuation and you’ll often hear the grammar mistakes.
However, you don’t want a reader coming across a blaring grammar mistake. They stop thinking about what you’re writing and start focusing on grammar. Someone compared it to walking across the floor and tripping on a coffee table. I, whom one can assume knows the rules, once wrote “Yesterday he lied down.” I heard about that. The past tense of lie is ‘lay.’ It should have been “Yesterday he lay down.”