Depends on the trip, but unless I have explicit constraints (e.g. first night hotel if arriving late via plane), I try to keep my options open. That is easier to do on a longer trip than a quick weekend trip or mini-tour.

What I have done is go to a site like motelguide.com and create my list of towns I know are likely to have motels. This way in the morning, I'll likely know the candidate places I could stay if I want to stay indoors. I haven't done same with campgrounds since mostly been fortunate to always find some place to camp.