Probably three things factor into what determines the importance of making advance reservations or winging it.
The destination and time of year may dictate your need to reserve a site. If you are heading to an extremely popular place in the middle of the summer travel season then maybe you would want an advance reservation.
Is your rig 24' long or are you a 45' DP pulling a 24' trailer? Back in sites for smaller rigs are much more available than 80' to 100' pull through sites. If you have certain length restriction maybe reservations would be in order.
Lastly, do you have any type of timetable to maintain? Is there a reunion in the middle of the trip you have to be at? Do you have to be back home at a certain date for work? If you are on a schedule then again, maybe reservations are in order.
So I guess like someone else already said, what fits your needs.
Full Time in 2017 Newmar Ventana 4369
pulling a 24' enclosed (Mini Cooper/Harley/Kayak)
(traded 2014 Thor Challenger)
US Army 70-73 Retired LEO