I think you will find (much like any question about
MVMCP) you will get a lot of varying opinions on this topic. I don't know what your situation is as far as having young children, but if you want to avoid a night that is sold out, I would wait until the night of the event. If you have park hoppers, I would spend the day at MK and wait until they start escorting people out. Then (as late as possible) go see if you can buy tickets. If it is sold out, you are better off than those that have already bought tickets and are about to find out what it is like with 25,000 people plus those that refuse to leave for $50 a ticket. If it is not sold out, then you can take a chance.
In addition to being able to avoid a sell out, you'll also be able to avoid wasting money on a bad weather night. That is my advice after having wasted $150 on MVMCP in Dec 2008.