The first thing you need to decide is whether you're going to go to the parks on the arrival and departure days. The incremental cost of a couple extra days is very small with the MYW tickets, but depending on how you're getting there, where you're staying and the exact times you're getting there and leaving, it may not be worth it for your peace of mind.
On our last trip (just a couple weeks ago), we arrived after noon on Saturday and left before the sun came up on Sunday a week later. We went to Epcot on the day we arrived. We found that we got a lot done in Future World in the afternoon, because by then a lot of people have moved on to the World Showcase. You may not get to do Test Track, Soarin' and Turtle Talk, but you can do most of the rest of Future World by closing time (or cut out earlier if you're tired). With 6 more days in the parks, we opted for another Epcot day, 1 day at MGM, 2 days at MK and 2 days at AK. The only reason we spent 2 days at AK (we usually only do 1) was that one of the kids was sick on the AK day and missed EE. Otherwise we might have chosen to do a third MK or Epcot day or, if we only had 6 days for the parks, the 2nd AK day is the one that would have been cut out. (My kids are 12, 9 and 6, by the way). We've been staying at Fort Wilderness for the past few years and we always plan at least one campground day each trip, and usually do it right in the middle.