I mean, it really is if you've configured ACME clients before, since this is THE parameter for choosing the CA. But indeed, not everyone will need it. That's why I think a link to getting started will be a good way to point people in the right direction, which you'll find in the mockup.
That'd probably help. I still like my idea though 
Let's Encrypt has exactly one interface for using it (not counting the fake one). It's a single URL. Just seems so important to me that it should be highly visible.