After finishing or achieving something great, one may be tempted to stay in the flow and start with the next thing. However I think taking a break is important.
You appreciate your achievements more if you take the time off. When just finished, you still have the work view of your result. You notice little things that could have been better. You may even be self-concious about it.
Only after taking some time off you see the project as a whole and are able to fully appreciate the work, even if not perfect. When you start to focus on the next thing immediately, you will always have the finished project in mind as "could have been better".