If you're using a dining plan, it's highly recommended to make ADRs beforehand. This is due to the fact that you absolutely need to eat at TS in order to make it worthwhile.
Otherwise, if there are any must-dos, or you are inflexible about time and location, it's a very good idea to make ADRs for those as well.
Now, if you're not on a dining plan, don't mind CS, are flexible about both time and location, you can certainly do it without ADRs. It may be best to check in the morning to see what's available rather than waiting until meal time to do so. Keep in mind that the chances of you getting into the popular places are going to be pretty slim though.