If it's a simple 5 minute change, why not implement it and use DI for a single instance? I imagine you would save a good amount of time by being able to run automated tests now rather than the manual tests you described in your other reply.
Apple makes it notoriously hard to do any kind of testing for ObjC/Cocoa apps, especially testing the GUI [1]. My solution is to only write really tiny apps, and manually test them. I still use TDD, I just make sure the app is small enough that the regression suite can be done manually in a minute or two.
[1]: I'm aware of third party solutions like Kiwi and Cedar, but I haven't found them to make this process much more pleasant than the built-in tools. Apple is really just an overbearing mom in this case, forcing you to use the sweater she chooses, and making life difficult if you don't.
