How far ahead to make reservations depends on a few things. First is how popular the areas you will be in are; if they're popular, reservations well in advance are often nearly essential to getting someplace reasonable to stay. Often you can get some idea for this from some online reservation systems that show how many sites have been reserved to date. If 90% of the sites are available, chances are pretty good they won't all fill up while you're en route.
The second thing is your personal preferences. Some people prefer to preplan things in detail and rest easy knowing that they have a site waiting for them (and get anxious with the uncertainty that comes from not having a reservation), while others prefer the freedom that spontaneity brings. Some find the planning nearly as much fun as the trip, others find it a chore. None of us can tell you how these things strike you.
A third consideration is how late you're going to be arriving at the campground. First come first serve sites are, rather obviously, more available when you're among the first comers. Earlier arrivals also mean more time to find an alternate site, if needed.
I tend to be somewhere in the middle, I guess. For shorter trips in particular, I usually make a reservation, but often not far at all in advance (maybe a couple of days). For my recent extended trip to Alaska, I only made two campground reservations in advance, one for Denali and one for Lake Louise--both quite popular destinations.
