The official reply is usually "when it's ready". It's great to tell clients about the upcoming features, but if you have a deadline then I'd be building on 2018.2 only.
Hi Chris, thanks for responding.
Yes, I've been asking around and that seems to be the answer, which is completely reasonable, as who knows what will come to light as it is tested by the user community. It feels like Tableau releases updates quite often, but I've never paid attention to what a typical time between a beta and production release is (and maybe there isn't a typical time). Thought someone might have a sense of it that would help my client decide whether to push their deadline. But we have moved forward with 2018.2.
And yes, I agree wholeheartedly with you about building in production. I wouldn't build something for a client in beta, we'd wait for a production release.