Perhaps not whether or not the belief is useful or not, as to whether we label it as irrational, but if that belief (for the past, developed in the past, useful in the past) is detrimental to us now, in the present. We do have many "beliefs" I think that were formed from the past, but because they no longer are "active" in the present but just "are" doesn't hurt our having them.

It's the ones that do injure us psychely... or even physically... that we have to counter. ?