Requirement for filing form 706

[Rickm]

A client whose spouse past away in 2021 has not filed a form 706. The joint assets total about $7M and the deceased individual's assets are about $3M. Is a 706 required to be filed in ordered to record the step-up in basis for the deceased's assets. Also, the personal residence was jointly owned. The assets are well below the exemption amount but are there other advantages to filing a 706 for the surviving spouse.

[RWG1950]

I'm pretty sure a 706 is not required for basis step-up, but since we don't know what the estate exemption will be after 2025, wouldn't couples
with an estate of this size want to file one if only to transfer the unused exemption to the surviving spouse?