If you are building a web service for the general public that you want to last forever (e.g. an interface to Wikipedia or something), follow theoretical REST principles and you'll be well served.
If you have actual paying customers that you'll need to support over many years then version your API. Paying customers won't tolerate API breakage and you can't always force them to update on your schedule. Assuming you still want to evolve the API and also sign up new customers, you'll quickly find out you'll need versioning.
If you have actual paying customers that you'll need to support over many years then version your API. Paying customers won't tolerate API breakage and you can't always force them to update on your schedule. Assuming you still want to evolve the API and also sign up new customers, you'll quickly find out you'll need versioning.