We've done it both ways. I have to say what works the best (for us) is minor planning. We like to be flexible. We tend to move more around the kids. If they are begging to do a park we didn't necessarily want to do that day, we still do it. If the kiddos are happy, we're much happier regardless.
Our planning is limited to ADR (PS) at must-eat-at restruants during regular and peak seasons; not so important during off season. And if we have ADR at a restruant in MK, but decided to spend the day at Epcot, we just jump the monorail. In other words, we plan on being at a certain restruant at a certain time, but don't find it imperative to spend the day at that place. It works nicely for us.
About the only other thing we "plan" on is checking things that may be closed during our trip. For example, our last trip we went to MK the night we got in in order to ride Peter Pan before it closed for refurbishment the next day. If it were an attraction like Hall of Presidents, it wouldn't matter at all.
OK, now I will admit a lot--A LOT-- of thought goes into deciding when we will go and where we will stay. That can make or break our vacation. I don't like to deal with too heavy of crowds or too much heat--no holiday or summers for us. And I want a hotel that has everything we may want (room service, just in case) and a nice atmosphere.