This is something I regularly do when composing a plan for a new book, and find it can be extremely helpful when writing up the initial rough draft of the manuscript. It really depends upon the kind of book one is writing, and how one feels when they are writing whether or not to include dialog in the plan. I find that some books are more along the lines of just plotting the action in the first instance, and segmenting the scenes into chapters or parts of a chapter without dialog.
Many times when writing the plan, I find myself carried away by certain scenes, and the dialog just flows from these character exchanges. By having scenes peppered with dialog in the plan, it can make the writing of the first draft so much simpler, as it can be a matter of copying these scenes into the rough draft, and using these as a starting point when refining the particular book in later stages.
If you are not certain about including dialog in your initial story plan, give it a try and see how it works for you. Every writer is different in the way they plan their stories, and execute the writing of these. If you are like me and dialog can crop up when least expected, run away with this if you feel it works, and expect to make yourself excited about the story, and readers in the process when you publish the final draft!