The time invested to strategically develop your marketing plan will absolutely pay off. By having a plan in place, you will be able to better plan your product and material orders, secure better rates on advertising and, usually, your product/material orders and relieve a significant amount of stress while trying to run (and grow) your business.
The fact that you're reading this article shows that you are wiser than most business professionals out there. Not having a plan is like packing for a vacation, without knowing if you're heading for the beach or the slopes. So congratulations on taking this first step.