We used to travel a lot with our 5th wheel, have crisscrossed both Canada and the USA several times.
Reservations for weekends are indeed recommended as all the happy campers will be out in full force and campgrounds fill up fast. In popular tourist areas (think Banff, Yellowstone, Mt. Rushmore, that sort of thing) if you don't make reservations weeks, sometimes months ahead, forget about it...even on weekdays.
But, being on a bike, you have some flexibility - if you find a campground full don't be afraid to ask for an overflow campsite, or even a glorified grass patch somewhere - without a fullsized vehicle and no RV they may find a spot you can crash in, trust me. You get more flies with honey vs vinegar though, remember.
As for winging it, use your head - choose wisely and chances are nobody will bother you.....so, camping in a public park in Downtown Detroit=bad, camping in an out of the way spot in a state forest = good. Yes, you won't have bathrooms and showers in the morning, but that's easily enough dealt with - truckstops are a ready made option where you can park safely and shower up before or after breakfast, and most campgrounds also offer day use rates will give you access to the facilities as well, but I'd favor state/federal parks over private parks in that situation as the day use rates are usually less - some private parks are nothing but cash cows and even a request to just come in and use their facilities or just empty holding tanks and pickup some fresh water apparently warranted a $20-$30 charge we often discovered. State parks, usually around $5, and although they might not
technically include access to the camping areas where you'll find the showers, nobody is going to bother you for going there and using them.
It all depends on how creative you are. We camped in MANY non traditional areas over the years, although we had the benefit of 4 hard walls and a locking door, so I'd be more careful when tenting, but I'd still do it in a heartbeat.