I struggle with this as well. We've been on the road full time for a year now and here's how I do it. If I'm going somewhere popular, then I break down and make the reservation. I really don't like to make reservations because plans are constantly changing and I often lose my deposit.
Someone mentioned that after Labor Day, they don't make reservations. That doesn't work in Florida. After Thanksgiving, reservations are usually required. So it really depends on the destination, not the calendar. If you are going to Fort Wilderness in Walt Disney World, reservations are required on virtually any day of the year except for weekdays in October or certain slow months.
A couple of years ago, I was staying at Anastasia State Park in St. Augustine on Labor Day weekend and I happen to be at the front office when a big MH pulled up with Western plates. The guy came in confidently and asked the rangers if they had any spots open for a few days. Of course they were booked solid so the guy asked about other campgrounds in the area. The ranger gave him a list and tried to explain to the guy that he needed to call the other campgrounds before visiting them but the guy just shrugged him off explaining that they've always found something this way. I was floored to imagine this guy was so absolutely clueless about Florida camping in season near the beach.
One way around reservations is to get good at boondocking. I use Days End and Harvest Hosts to supplement the usual list of suitable boondocking locations found in Allstays. But when we travel out East, we don't have the same opportunities for long term boondocking as those out west. Currently, I'm camped in my brother's driveway near Bethlehem, PA.
In July, I'm heading to Bar Harbor, ME. Last month, I snagged the ONLY remaining spot at a particular campground and am very happy I made the reservation two months ahead of time. Even then, I had to modify my arrival by two days.
So there is no hard and fast rule here. Just analyze the destination, calendar, known events and importance of getting a guaranteed spot. Lot's of full timers sit in one spot year round or hardly move. For them, this is not a problem. But for full timers who move frequently and like to chase the sun, it can be a little stressful.