Ensuring your writing is grammatically correct can be a huge irritation and a time-eating monster, or it can be costly to pay someone to do it for you. Do you really need to make sure each and every sentence complies with grammar rules?
YES, YOU DO! At least, you do if you want to be taken seriously as an author or screenwriter. The reason for grammar (and punctuation) rules is for clarity. Without them, what we think we’ve written could be read as something entirely different. Here’s an example of why the good old Oxford comma is important (contentious, I know);
“We had a party with the dogs, Taylor Swift and Justin Beiber.”
Or, “We had a party with the dogs, Taylor Swift, and Justin Beiber.” Note the additional comma.
The first sentence can be read that Taylor Swift and Justin Beiber are dogs, while the second one clearly states that the two pop stars were at the party with the dogs. Of course, if what you meant was to express your opinion of the two pop stars, then the first sentence is entirely correct.