We went in mid-October last year, and it was pretty busy. Avoid the weekend of Columbus Day.
The basic strategy is to avoid the MK on non-party days. If you want to see the Electrical Parade/Fireworks, show up late in the day to catch those.
If you do one of the Halloween Parties, most rides are a walk on. Even Mine Train will have a much shorter wait than usual, because no fast passers slow down the line.