You don't.

You use the 5498 to confirm that the client (and their broker) did in fact put their "stated to you" amount into the "type of account" as they are telling you, so that you have done your due diligence on that item. You will use FMV when it applies for purposes of IRA conversion computation, for instance, when there also is basis. Otherwise, that is an Information form and it is not part of a tax return.