There's only one setting that is unilaterally conservative in this business - reducing allowable stress.

In all other cases, the status of any other setting being conservative will be wholly contingent on which set of blinders you put on.

I'll preface my further statements that I'm largely unfamiliar with B31.1's requirements - so I'm assuming it's non-specific on the subject - and I'm also assuming your client and company policies are also non-specific.

But if (since) it's unclear which is more conservative, it's unclear to me why you wouldn't consider the entire pressure range in addition to the temperature range, aside from the multiplicative nature of combining load cases.