While the above advice is good (about certain times of the day usually being cheaper) did you consider hotel costs?
We usually fly out Saturday early AM and return the next Saturday late PM. Usually those flights are slightly more expensive due to the times, but we like to have an almost full first and last day in the parks. Yes it is usually cheaper for us to leave late Friday evening but then we have to worry about an extra nights hotel/parking. Or if we leave later Saturday we lose a whole day in the parks. Same thing from the return. It is usually cheaper to leave Saturday early AM, but again we lose a whole day in the parks. Or if we leave Sunday AM, we need to pay for a hotel another night.
So, to make a long story short, we look at the value of getting the exact flight times we want in more than just monetary costs. Now if we were saving hundreds of dollars, it might cause us to rething plans, but for $50 or $60 bucks, I would rather stick to our preferred schedule.