I can tell you, DH and I have done it both ways. (We haven't taken our dd yet, so I don't know how it is with kids!) I can say, the first time I planned.
And planned.
And planned.
And planned some more.
And when I was done planning, I went through everything to make sure I had planned it all right.
I was neurotic! But, when we got to wdw, about 1/4 of our plans went out the window, because we were doing other things. However, I can say that in most cases, it helped us a lot in just making things easier. We didn't wait in line as much and we saw everything we wanted to see and do. DH made fun of me for my planning, but in the end, we both agreed, it was helpful, and we didn't feel the least bad about ditching some of our plans - that's why we planned in the first place!
The second time around was a bit different. We were there on a trip with DH work, and I was so busy before we left that we just decided to see what would happen if we "wing it." No ps's. No park itinararies. We didn't even know the hours of the park before we got down there!
That second trip wasn't as fun. We waited in lines more, and we didn't get to everything that we had in our last trip, and we were there for the same amount of time - and staying on property instead of off like the first time!!! We felt a bit more stressed and kind of lost. Was it the lack of planning? DH likes to think so!
Can you wing it? Absolutely. Will it work? It depends. If you are okay with being flexible, and not worried about leaving wdw without seeing everything you set your sights on, then it's fine. Would I do it again? Probably not...especially since the next time we go, there will be kids in tow!!
Good luck in whatever you decide - I'm sure you'll have a great time!!