For me, it depends on the situation.
Many of my trips are pretty short due to work constraints, and for those I like to have reservations before I leave as there isn't much margin in the schedule. That doesn't mean the reservations get made months and months in advance; often the whole thing is relatively spur-of-the-moment, only planned a week or so in advance. Often these reservations turn out to be not really needed, as I pull into my reserved site in a half-empty campground, but I do appreciate knowing that the site will be there for me regardless.
For my longer trip to Alaska, I made very few reservations, as I could afford to (and indeed needed to) be more flexible. The few locations where I did make reservations were done so because they were necessary: Lake Louise, Denali NP, places like that...and even those were made not too far in advance.