I'm all in favor of correct grammar in fanfic. However, there are times when the author shouldn't use it. For example, I wrote a LOT of stories and poems based on The Crow: Stairway to Heaven (TV series back in the late '90s). To really capture the characters, I realized I would have to write the dialogue as the characters spoke on TV. This meant throwing out some of the basic rules of grammar, and it drove me crazy. The results were worth it, though. The stories (and characterization) ring true for me.