Don't use the handicapped stall, if:
- Someone in the restroom clearly needs it more than you do, or
- You have observed several handicapped persons in your current location, and suspect one might rush in with a mighty need to pee while you're still doing your business, or
- The handicapped stall is filthy, out of order, or already occupied
Otherwise, have at. Just please make sure you don't leave a mess behind you -- in short, follow standard restroom courtesy.
Modern U.S. society has embraced the idea that handicapped persons should have special, easier-to-use facilities readily available to them.
Some facilities, such as handicapped parking spaces, are intended exclusively for handicapped use, and their use by non-handicapped persons can result in a fine.
However, restroom stalls are not intended for the exclusive use by handicapped persons -- one is supposed to immediately make them available to a handicapped person when possible, but otherwise, they are to be treated as any other toilet stall.
Why the difference? Because parking spaces and toilet stalls are fundamentally different facilities.
Parking spaces are for parking your car or other vehicle. Some lots provide for short-term spaces (like the 15-minute spots by my local post office) and some lots place limits on how long one can park (like the no-overnight rule in the garages at my university). But in most lots people can park for quite a long time, often hours. Even a quick trip into a store can result in a car being parked for more than 15 minutes.
And in the car-loving U.S., parking lots are vast, and traversing them can take quite some time.
Handicap parking spaces are intended so that people with limited mobility can more easily access nearby buildings. Many people who do not need wheelchairs have handicapped plates because although they can walk and stand, they can't walk far without pain or exhaustion.
Parking spaces in busy locations are always at a premium, and Americans are often least courteous when they experience the frustration of trying to compete for a space. Thus, it works out best for everyone if the handicap spaces are permanently reserved, and have posted fine warnings to keep them free.
Toilet stalls are intended to provide privacy for attending to bodily functions such as elimination, changing sanitary napkins, etc. Sometimes people use stalls to change clothes. Some use them to quietly weep. In general, though, one can attend to business in a public restroom stall in less than two minutes.
Thus, the wait for a specific restroom stall, except in terribly crowded festival conditions, is going to be far less than the wait for a specific parking space. A person who enters a restroom to find their desired stall taken is will not likely have to wait more than a minute or two for it to become free for use. And presumably, a person who cannot hold their bowels or bladder for a minute or two has already been outfitted with a pair of Depends.
Handicapped toilet stalls are intended to provide space and handholds for people in wheelchairs, certainly. But the extra space is also intended to provide room for a caregiver to help another use the facilities.
It is on that last point that handicapped stalls cease being intended solely for handicapped people. Most handicapped stalls in women's rooms are the location for baby changing stations -- the stalls serve double-duty for parents with babies or young children who can't yet go potty by themselves.
Furthermore, many restrooms that have been retrofitted to accomodate handicapped stalls have consequently reduced the size of the existing stalls, making them physically or psychologically uncomfortable for larger people to use.
So, when faced with an empty restroom that contains several tiny, cramped stalls and one spacious stall, should an able-bodied, responsible restroom user feel guilty for availing him or herself of the more comfortable choice? I don't think so.
So, my take on this is that you can use whatever stall you feel like using as long as you use it with courtesy towards anyone who might wish to use the stall after you.