OK it depends,
1) For major holidays: 4th of July and such, you better have reservations everywhere.
2) For major National Parks, Yosemite and such, you better get them and as far head as you can. Yosemite is actually a bad example it is so popular they have a lottery system. Yellowstone is better example, better make reservations there the winter before you want to go.
3) For commercial parks you can often do without. When we travel we don't make reservations and just call a hour or two ahead to get a spot. This rarely fails.
4) For popular park systems, Florida State Parks and from what you say Oregon parks, you better have them and maybe will need them far ahead. For less popular parks systems this may not be true, we can usually get a spot in Georgia State Parks for example and it seems like Texas parks may be the same.
So it comes down to timing and location. You can usually find a commercial park but if you want a specific NP or SP better plan ahead.
