Our trips are about half planned, half spontanious.
I think if you have realistic expectations, it's fine. Know that you may not be able to eat a table service meal where and when you want. Know that you may either not be able to ride headliner rides and if you do, you may have to wait a long time for them.
We have never waited until the morning to decide what park, only because my son (7) does better with a bit of structure, so knowing we'll go to Epcot and ride The Seas first (for example) keeps him in a better mood. I also often let him lead the way (though I do help him with decisions like wait 20 minutes now or maybe come back and wait longer or miss it all together).
I also feel like I have enough knowledge of WDW to know that my spontaneous decisions are good ones. I do tend to avoid MK on Monday's because I feel like it's more crowded, for example. I know riding certain rides during parades could decrease wait times, etc.