Depends on the Project
It will depend a lot on both the project and your style of work.
For me for roughly 90% of my projects I can plan out the project into excruciating detail and if I do the planning up front it's not going to change over the life of the project. I also work with very long term projects, something that is not common in GTD. So I tend to plan everything out as far as I am able and use a GTD tool that helps me by only presenting onto my action lists the actual clear next action I need to do.
For other projects, they change regularly, what you thought was the sequence of actions turns out not to be, the goal changes, or other stuff interferes with the plan as you originally thought it out. For those projects planning beyond the single next action is a waste of time.
Only you will know what type of projects you usually have.
My suggestion is to take one project, plan it as completely as possible and see if you are able to use that plan without changes. If you can and you like the way it works for you to have everything laid out then go ahead and plan everything but do not put more than the next action on your lists. The remainder of the plan is project support material unless you do your project support within the GTD tool you use.
The concern is you should rarely have more than one action that is related to a single project unless they are totally independent of each other. In my case most of he time when I find independent actions for a single project, what I really am finding is a hidden project within a larger area of focus.
Oogie McGuire - Mac, iPhone & Omnifocus
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Paonia, CO USA