I think it is perfectly appropriate to use the disabled stall if it's free. It's the same thing with hotel rooms with disabled access/modified showers. Hotels don't assign those rooms only to disabled people. I once stayed in a hotel that wasn't even occupied close to capacity and didn't make any kind of room request. They just assigned me the disabled room for no particular reason. I see no problem with it if it is available.
Even if a disabled person came in while you were using the disabled access bathroom stall, it's no different than a regular person waiting for a regular stall because they're all occupied. Everyone has to wait their turn - no matter their circumstances in life. My father, MHRIP, was disabled shortly before his death, and was wheelchair bound briefly. He never complained about waiting for a disabled stall, even though sometimes he would need to wait a minute. Those stalls are there to be able to accommodate people with special needs; however, they are not there solely to cater to them. My father would have been offended to be given "special" treatment simply because he was sick; he was still himself; he didn't need special treatment, just access to modified facilities. Access is access, even if one has to wait for a minute. There's no law that says that ADA compliant restrooms can only be used by those with disabilities.
Or, consider this. A lot of public and government buildings have disabled-access door buttons that one pushes to get the swinging door to open automatically. A lot of non-disabled people us it, especially at stores and libraries, when people have their hands full with items they've bought or checked out. And, sometimes people are just lazy and like to use the button instead of taking a little bit of effort to push open the door. That is not illegal. Imagine if the only entrance to a building had that disabled access feature. Would non-disabled people be dis-allowed from entering the building? Of course not! That would be silly. All it means when there are disabled-access facilities is that thank goodness disabled people can use them, too. It does not by any means indicate that non-disabled people can't use them.
There IS a law about parking spaces, though. And to use those, the disabled have to have a placard supplied by the authorities, and it is illegal to use one that isn't yours. I see no requirement about bringing one's placard/license to the restroom and hanging it on the door while one is using it. Until that day comes, if it ever does, feel free to use ADA compliant facilities where it is legal to do so, is my opinion.