I'd say if there's something that you consider a must do, then make the PS before you get to DL. I don't think you'll have to call the very day the PS window opens, a month or so before is probably OK.
Blue Bayou for example, if you want to go here, I'd say definitely make the PS before you get to DL. I have went to other places with no PS's made (Cafe Orleans for breakfast and Tortilla Joe's for an early dinner are two I can think of off hand). Of course we are very flexible and it's just DH and I so we eat when we are hungry. Sometimes we have called the day of and get one for later that night.
Another thing it would depend on is when you are going. For instance if you are going Christmas week, I'd make one for every place I wanted to eat while I was there. If you are going at an off peak time, say middle of the week in Sept, you're probably going to be able to get in fairly easy. Being flexible with times also makes it easier.
I like picking a couple places and making the PS's before we go, it makes the trip seem that much closer!
HTH!