For the IdP? Sure. That's the point. You just have prior knowledge about what might happen, so you can save it the trouble. If signing is a "whatever" sort of operation to the IdP, the logical thing to do is to sign if the flag is true, and do whatever the default is if it's not. If it's a major operation that the IdP doesn't normally like to do, then you'd probably consider returning an error.

If it was written as a MUST (in which case that would be a bad name to use), I'd follow it, otherwise I'd do whatever I'm doing now.

-- Scott