Voiding is a GREAT option if you know that you performed significantly below your potential. I would always void if:
a) you do not essentially complete any passages in a section. You almost always have to complete the MCAT sections to get 10s. You really want 10s to get into med school. This does not mean missing one or two questions but DOES mean having to christmas tree or near-christmas tree a passage, IMHO.
b) you know that you got LARGE NUMBERS OF QUESTIONS wrong -- a much larger number than you usually do. Basically, you know you BOMBED the exam. Since this is obviously a much more subjective rationale than (a), I would be more cautious about using it.
c) you get a severe headache, etc, during the exam that severely effects your performance.
The key here is to avoid having a score that is significantly outside of your normal range on your MCAT record. Remember that your MCAT record is essentially permanent.
A lot of schools will be forgiving if you have one bad MCAT score and do much better on a second one BUT they will expect some improvement and account for that. However, few schools are as forgiving of third of fourth MCAT scores. I really don't think you should put yourself in the position of having to overcome a bad MCAT score if you can avoid such a situation. Some of said situations can be avoided with careful use of the void option.
Note that if you void, no one will even know that you took the MCAT. You can just pretend it never happened, learn from any severe mistakes, and redo.
With the numerous MCAT dates available, you may be able to retake the exam in as few as two weeks so you won't lose your preperatory edge.
Sorry for the long rant there.... got a bit carried away
. (Also, don't void if you did horribly on the writing section because no one cares about that.)