We've tried a number of different approaches during our visits with the girls...
2010- Our flight arrived at 11am and we hit MK as soon as we had everything situated at our resort. We ended the night with dinner at Chef Mickey's and fireworks from our room balcony. The only real break time we took was longer naps or sleeping in one morning.
2011- This was a road trip. We didn't arrive at WDW until 7am on day 3, but right after check-in, we hit our pre-park opening ADR at Crystal Palace and spent the day/night/EMH at MK - minus a nap break in the afternoon at the resort. We did have non-park days built into the end of this trip.
2012- This was another road trip and this time we drove non-stop (started in the evening hours). We arrived around 3-4pm and after settling into our room, we went for ice cream, walked around DTD, and then had a late dinner at T-Rex. We had break days built into the beginning of this trip and also spent time with family after a week at WDW.
2013- Well, this certainly didn't go as planned.
Still, even though it didn't go as planned- we still didn't jump into any of the parks until nt 2 of our trip.
I don't think there's any right or wrong way to approach it. I think some of it may be the overall stress of a trip and exposure to tons of germs while there that can cause trouble.