Planning improves efficiency, productivity, and effectiveness. Thinking it through before acting, looking before you leap, is almost always a great idea.
The level of planning required depends on the complexity of the project. Simple projects need simple plans; massive projects require detailed and thorough plans.
If you do not have a lot of experience in the area of the project, you are well advised to factor in a lot of contingency time and resources. You are wise to expect the unexpected and prepared to be surprised.