Ironically, the writers that worry about punctuation are most often beginners. Why? My guess is, writing from our school days was all about punctuation and capitalization and grammar and spelling and complete sentences. In other words, mechanics seemed to be the Biggest Thing.
In fiction, however, mechanics finishes dead last to story, characters, setting, and theme. Consequently, while an author shouldn’t ignore mechanics, paying meticulous attention to the rules of the writing road is premature if someone hasn’t given him feedback about the major elements that make up a story.
Why would a writer sweat over whether or not to put a comma before the and in a compound sentence if there’s a fair chance he may need to rewrite the sentence as two simple sentences, or as a complex sentence? Or as a deleted sentence. 😉
First the narrative needs to get down on paper or into the computer. Then the story needs a thorough revision, followed by a good amount of writing revision in which you pretty up your prose.
Certainly fix the mechanical issues as you see them, but a real search for those problems should not come until you’re ready to send off the manuscript to an agent who has requested the complete. At that point, you want your work to shine, and the other elements will be in place so you can concentrate on those irritating incidentals.