I think we are going to try to go Oct 10-14th.
In 2008 we went the first weekend of October and it was dead until Friday, when you get weekend crowds, plus it was gay days. So all in all we felt the crowds weren't bad, very do-able in on the weekend.
The following year (2009) we went the last couple days of September with Oct 1st being our last day in the parks. The entire week was SO busy! We couldn't believe the difference from the year before, where a few dates overlapped from year to year. They had limited fast pass options, as it was explained to me by a CM that if they don't expect the crowds they limit the fastpass use. So lines were very long. They ended up extending park hours almost every night we were there, which they did on a day by day basis and didn't announce until about 5pm.
So we have seen two very different views of this time frame. I was wondering if it got more crowded the closer to Halloween, but from all I've looked at, it seems to be fairly even, though Halloween day does seem to be very busy.
This is just my intrepretation after reading through other past threads and trip reports. I figured it can't really be any worse than the 2009 crowds we had, which were more like summer crowds (Pre cars-land summer!

)
Fall is our favorite time of year to go after trying every season, and at least there is always that chance for light crowds...