Words are just as important to success of your manuscript as plot and characterization. Knowing which words to choose can be a big boost to your career. Why say ran when you can say barreled, plunged, rushed? Don't pick the first words off the top of your head. Don't be satisfied with typical. Whenever you feel a lull in the flow of your book, look at the overall plot, etc--and then look at your choice of words. Are they vivid? Fresh? Do they evoke the right picture in your reader's head?
It's true that you could spend a lifetime polishing your manuscript and forget to get it out the door. Obviously, you should take a breath at some point and send it out--but be sure it's the very best you can make it, right down to the words you put on the page.