Could vary tremendously. For southern and coastal Fl. state parks reservations are a must IF you can even get them. Generally same for heavily visited national parks but there are some really big campgrounds in both Grand Teton and Yellowstone which usually don't fill. For parts of the country not coastal or heavily visited, just wing it. But, there can be unexpected fishing tournaments or big events which change everything. Only experience can answer your question but reservations remove a lot of stress for me.
