From my observation and discussions with other boaters, many of them more experienced than me...
The majority of people rely on their strongest roll in combat situations.
Playboaters who learn to roll on both sides and learn to "feel" which side is more conducive to rolling can have a real advantage in river running if they can go to either side intuitively in critical situations.
I work on my off-side roll and hand rolls, etc, but when I am in serious water (or really any combat roll situation), I still go for my strong side, even if it might not be the best side to roll on in that situation. That is one area I personally hope to improve in. So I practice a variety of rolls from time to time. I should practice more. However, I think it is fair to say the average boater does not practice their off-side roll.
There is no shame in swimming in the park.
Even if you didn't want to learn how to playboat, it is a great place to experience getting flipped for working on combat rolls. I swam a couple of times in parks and I think everyone has when they were beginners, if they were brave enough to go into parks before they progressed very far. It is generally a safer place to swim. And anyone making fun of a swimmer in the park embarrasses themself by being an idiot, more than I would be embarrassed by being the swimmer. Actually, you are more likely to have people quickly and politely coming to your aid than you are to have anyone being disrespectful because you swam.
I am sure others will have good suggestions for starting points.
Salida's lower wave was my first play park experience, and I found it amazingly gentle and inviting.
Paddle Day 22 - First Day in Salida
The hole at Lawson is probably the friendliest versatile hole I have encountered. I am not much of a playboater, but those are a couple of places I hope to spend more time in for my own limited playboating progression.
Here is one of my more interesting days of paddling, and why learning an off-side roll might be worth considering. I still wish I could roll as good as this guy Ryan, who was able to save the day because of his rolling expertise.
Paddle Day 40 - Importance of Off-Side Rolls