It really depends on when you go and where you want to eat. If you are going in a period that is very busy (summer, holidays) or during a free dining period, you really should make them for most or all of the TS you want.
There are some places to get a meal without a ressie, but not many of them are in the parks. Also, if you don't have one, you will have much more luck at odd times, like 4 pm dinner or 10:45 am breakfast.