You would have to further define your terms, but in the case of dialogue (or the spoken word) as a part of diction, grammar is important only to the degree of accuracy required to advance the comprehension of the work.
Hi O-uknow~ I would suspect it depends on the audience. All languages, as a general rule, are subject to change. English is one of the most frustrating because it dictates exceptions. If you are speaking about how people address one another in verbal form and then adhering that same style into written communication, we have found ourselves in a whirlwind of trouble. I am a terrible example or a great example. It depends on how you look at it. I was raised to speak correctly. Not, "Yo. I ain't no back ass hick just goin w y'all, Ya here me sayin? Ya know, right?" You can wrap your head around this, ya know, right.?" I can understand that better spoken than written, ya know, right?. As the generations change the language, I find it more difficult to read. I do think it is important to adhere to some of the basic grammar, punctuation, and spelling rules. Syntax makes it easier to read and to listen to. In conclusion, It would be nice if you could wrap your head around this fundamental problem I have. I still don't get the need for reassurance by adding the word "right" after a person is already asking, "you know." Isn't that showing that the person asking isn't really certain about what they are saying? What is the point of saying it, twice? How much reassurance can the recipient give before it becomes insulting? I would like to find a happy medium.
It's situational, so there's no easy, quick, definitive or all-inclusive answer. There are some instances in which one is more important than the other, just as there are instances in which they are equally important. ~