I have only ever asked permission once in all the years that I've been using all of the above.
One Cracker Barrel in the middle of town that had no signs forbidding it. It was the first time I'd ever stopped at a Cracker Barrel. BTW, they said yes.
I regularly use W/Ms, Flying J/Pilot, Casinos, and rest areas.
BUT I also use Allstays.com to research which W/Ms. I have the Flying J/Pilot RV guide and use it to tell me which ones have RV parking. Even if I don't need the length of the RV parking spots it lets me know which lots are big enough to have a quiet spot to park. I use Casino Camper -
http://www.casinocamper.com/rv-parking-locations to find which casinos are okay to park, and which ones either don't allow it or charge for overnight stays. Cracker Barrel also puts out a location guide and can be found on line as well but there is no listing included of which ones are okay so I tend not to bother with them because I can't find out ahead of time. For rest areas, the information can be a little harder to find because a lot of states and provinces do not publish their overnight stopping regulations. Here's the website that I use to check on rest areas -
http://www.frugal-rv-travel.com/rest-stops.html
It doesn't matter what you're in. I've seen as many overnighting in cars at Flying J as I've seen big rigs and I've used them all when travelling in anything from a Plymouth Grand Voyageur to a 35' Class A.
The most important things is that you don't overstay your welcome, obey any rules posted, keep your vehicle and the area neat and tidy and be discreet about it...lawn chairs, barbecues and stuff left outside your vehicle overnight is not the way to keep us all on the good side of the merchants who kindly allow us to stay!!