For me, there is no "if." Effective writing does involve planning, and the implications for instruction are many. When we teach writing, we teach it as a process, not simply as a product. Most instructors explain writing as a series of stages, and recursive stages at that. In order to get to effective writing, the stages create a kind of scaffolding process. Traditionally, most teachers required students to create outlines before they proceeded to write, but today we know that there is even more to the process than that. We can look at some of the specific implications in the classroom.
Freewriting is a legitimate activity for the classroom, a warming up exercise, an activity from which meaning might emerge from chaos. Students need not plan at this stage, but this is a valuable way to get students going.
From freewriting and/or brainstorming, ideas will emerge, at least I hope so! This is the stage at which some planning must occur. Does the idea need to be limited or expanded? What can the student say to support the idea? Is research necessary? What kind of research? This all involves making some plans.
Once an idea is refined, the student needs to be able to articulate what points he or she wishes to make to explain, support, categorize, compare and contrast, or persuade. At this stage of planning, an outline is a handy tool, but since many students have "outlinephobia," I often talk about making lists of points. The outline is not a sacred tool.
A draft comes next, of course, based upon the plans that the student has made. At this point, writing becomes a strongly recursive process, since a draft might result in going back to the drawing board, doing research, pursuing what appeared to be a byway originally, and so on.
From the draft stage on, an emphasis on revision is central to instruction. There is a wonderful chapter in Anne Lamont's book, Bird by Bird, which is entitled "Shitty First Drafts." I have shared bits and pieces of that chapter with my students, as well as providing them with examples of famous writers' drafts.
Even the best and the brightest of us cannot write effectively without planning, and it is up to the instructor to model and facilitate this process. I frequently assign myself a paper and share my planning with my students, hoping they will see that even the teacher needs to brainstorm, plan, and revise.