If the project is critically important, then you'll probably end up working on it all the time, so scheduling would only serve to help you make sure you're doing the most important things first. In this mode it's all hacking, you're just putting tasks in order.

Otherwise, scheduling is a very good idea for different reasons. If it's a personal project, which means there's no one else to goad you into working on it, then it's especially important to use scheduling to help you keep working. Remember what PG says about personal projects: Always produce. Do something everyday.