There can be "stress" going with OR without reservations. While you may sometimes have to choose a different campground because the one you planned on was full when traveling without reservations, there is just as much stress having to pass up something interesting so you can get to the campground you have reserved for the night. I can't tell you how many festivals, park concerts, and other surprises I've discovered while traveling that kept me in an area longer than expected.
I've spent almost 800 nights in the trailer over the last 4 1/2 years, most 1 to 3 night stays. I rarely make reservations - special parks (such as Teklanika Campground in Denali National Park) or holiday weekends are about the only time I make them. If I'm concerned about a location, I'll call campgrounds in the morning to check if sites are available, and sometimes make a reservation for that night if they are close to full.
I've been doing this for many years, and have never had to stay in a Walmart, etc. I do stop for the day by 2:00; if you plan to arrive at a campground late in the day or at night, going without reservations is going to be a bit more difficult.
I have had to stay in a commercial campground rather than a state park on the coast of Oregon during the middle of the summer, but often first come sites are available, even after the "Campground Full" signs go up.