Correct document size saves you money
Saving money is so important, whether you work for a studio or as a freelancer. Prove to your art director that you can be frugal and deliver a project on-budget. Prove to yourself you can be frugal and put more money in your pocket. Now, you may ask, what are you talking about? What's the relationship between my creative and money?
As an independent or freelance designer, many times the only person you can rely is yourself. Hire a production artist to complete your project? Maybe, maybe not. The project budget may not allow you to take on outside help. You can rely on your print vendor to preflight your files but once you've released files, you want to be able to say, "boy, glad that's done!" and move on to the next project.
Makes sense, right? But many times, both as a production artist and a prepress operator, I received design files that were the wrong size.
But During the creative process, if your not used to working this way, that's fine but strive to deliver final art at the final trim size of your delivered printed piece.