GPS data varies wildly in accuracy depending on location.

In big cities, GPS can be very inaccurate because of the urban valley effect and the resulting multipath interference. Often, the best location data comes from proximity to Wi-Fi at that point, which can get you moderately close to the right place, but it still won't be all that accurate.

Similarly, if you're next to a vertical cliff (under it), GPS can be off by tens of feet because of reflections off of the cliff, and in that case, Wi-Fi doesn't help you at all.

The accuracy also depends on whether a given app asks for precise coordinates or not. The Camera app probably asks, but whether it waits for sufficient accuracy or not is another question. (It probably doesn't.) You might find that taking a shot and then taking another shot a minute or two later gives you better GPS accuracy on the second shot.

Either way, it sounds like what's critical for you is knowing the current accuracy radius before you take the shot. I don't know if the iOS camera app embeds the radius information or not, but I'm pretty sure there's no way to make it show it. However, it is possible to obtain the GPS accuracy radius from CoreLocation on iOS, so it should be trivial to write an app that shows it on your screen and warns you if you take a picture when the accuracy is less than your desired accuracy. (This, of course, falls more on the programming side of Stack Overflow.)

It's also worth mentioning at this point that CoreLocation will provide a location even if it does not have a GPS fix. The error radius of a Wi-Fi-only fix is bad. The error radius of a cell-tower-only fix is even worse. So unless you're regularly switching back and forth between a mapping app (that shows the radius) and the camera app, you'll have a hard time guaranteeing an accuracy better than the few miles offered by cell tower triangulation without writing a custom app.