I'd like to change the location that screenshots created via File > Save Screen Shot in the iOS Simulator are saved from the desktop to a folder of my choice. How can this be done? Perhaps there's a user defaults value I could change, much like the one specifying the location of standard screenshots on OS X?
In Xcode 12 we now always set the default location to wherever you last saved a screenshot, so to change the default location press ⌥ + ⌘ + S then select the location.
Also new to Xcode 12 you can right-click on the screenshot preview window that appears and you should see Desktop, Documents, and your custom location (if you've set one that isn't the Desktop). So even if the default is somewhere else you can always send the screenshot to the Desktop. You can also drag this window and drop it in an app or the Finder.
In Xcode 9 you can hold down Option while saving a screenshot and we will prompt you for the location. Check the "Use this as the default location" box to make the selected directory the default for all screenshots.