I second the previous posters advice of planning around your tentative parks- just so you are in the area - Though I tended to AVOID the EMH parks in my planning. DD(5) hates lines so I am trying as best we can to avoid the "busy" parks of each day.
I can't speak to booking same day but I liked the aspect of having an ADR around the park we intend to visit well in advance and at least having some degree of structure. Course the plan could be shot out of the water pretty easy once we get there.
I would just fear by winging it that I'd end up with extra TS credits at the end which would be a waste.