If you really do not care what restaurant you dine at (even to the point of having to leave the park and go to a resort) to eat dinner, you will most likely get in somewhere. Many times Epcot TS and MK TS are completely gone, even by the time the park opens. When I got into MK on Dec 27, there was a sign stating that all TS restaurants for the day were full.
If you have an idea of places you'd like to go and just don't like the idea of being tied down, I would really try to make some ADRs, at least for park days or for places you'd like to try.
Or, if you want to wing it, you could wait until you get into the park that day and try to make a dinner ADR in the am. If they're completely full, which is entirely possible, then you know to plan to be in DTD or at a resort for dinner and you can try and make an ADR for that location instead.
If you really don't want to do ADR's at all, not even the day of, you should be prepared to wait for a table and possibly be closed out of the more popular places.
You said you're not sure when you'll be hungry. Why not make a dinner ADR around the time you normally eat dinner at home? If you're hungry earlier, you can always get a snack or show up a little early for the ADR.