I've read that too and I know when my kids were little we always started at MK. In July when we went with my brother and family for thier first time, although I've followed the guide books in many things, I opted not to take this peice of advice. I started in MK, to kids (of all ages) MK IS WDW!!! I think the first step someone takes in a park should be the "magic" of MK, on Main Street, with the castle right in front of them. The looks on their faces is priceless.
To make kids wait until the "end of the week" to go to the place they REALLY want to go to me doesn't make sense. We went the first day for EMH, did Space Mountain and Buzz first, then had a 10:00 reservation for breakfast at the castle. My SIL said, that she couldn't think of a better way to start the week. It was almost like my niece (age 7) had seen Cinderella, her main wish, and was willing to move on to all the other things we had planned, instead of whining the whole week about "when will we go to MK and see Cinderella!!!".
After that we did some more rides, and then I had planned an experience with the lady from "Gifts of a Lifetime". It was VERY neat. Now THAT"S starting things out with a "bang", and nobody seemed disappointed about the rest of the week.
We then went back to the resort for afternoon pool time and ended the day at Epcot with dinner at the Biergarten and Illuminations. We of course went back to MK for a different evening a couple days later and they got to "wander" around doing rides while we saved seats for them for the parade, so they were there a couple different days.
I thought it would be cool to end the trip in MK too, but that was our AK time and we had such a relaxing time in AK that day and our trip ended with the Festival of the Lion King and late lunch at Pizzafari. Everyone agreed that was perfect.