Trading plan is a must for every trader, I think.
All the operations with money on the market should be sober and logical. I doubt that it is possible to make a logical decision without through planning because it requires some work to account all of the factors and work out some solution to the problem.
I, for one, cannot even imagine trading without a plan. I mean how will you know when to close the deal or where to put your stop loss? If you are going to work off the top of the head, then you are highly likely to be influenced by your emotions and lose everything that is possible. So, I strangly recommend to plan all of your actions while trading because the plan can bring you either profits or experience when you can track down the possible mistakes in your plan and make some alterations and modifications.