Is there any danger in using ConfigureAwait(false) in WebApi or MVC controllers?

it seems like I should use ConfigureAwait(false) on ALL of my async calls that are not tied directly to the UI.

Not quite. That guideline doesn't make sense here, since there is no UI thread.

The parameter passed to ConfigureAwait is continueOnCapturedContext, which explains more clearly the scenario. You want to use ConfigureAwait(false) whenever the rest of that async method does not depend on the current context.

In ASP.NET 4.x, the "context" is the request context, which includes things like HttpContext.Current and culture. Also - and this is the undocumented part - a lot of the ASP.NET helper methods do depend on the request context.

(Side note: ASP.NET Core no longer has a "context")

should I be using .ConfigureAwait(false) on all of the above await calls?

I haven't heard any firm guidance on this, but I suspect it's OK.

In my own code, I never use ConfigureAwait(false) in my controller action methods, so that they complete already within the request context. It just seems more right to me.

If there's no actual context in a ASP.NET Core application, it should do no harm nor good to add .ConfigureAwait(false) to your awaitable methods into controllers.

However, if there is a chance that eventually in the future, for whatever reason, there's something like a context to be taken into account as in ASP.NET 4, that would be a different story. We could not risk running in a different context, unless we don't give a damn about it (in which case we could use whatever thread is available for processing, thus possibly improving performance).

My choice here is to add ConfigureAwait(false) even if it's not used.