To clarify, the alternate 6 month valuation is an election per section 2032 made with a timely filed form 706.
In order to use the alternate valuation date, the value of the gross estate must have decreased since the date of death. Therefore an appraisal must also be made at DOD to determine if there is a decrease in value. The only practical purpose for the election is to decrease the amount of estate taxed owed.
When the election is made, any assets disposed within 6 months of date of death are valued at date of disposition; all other assets are valued at 6 months from date of death.
If the section 2032 valuation is used, then the basis for gain or loss is determined at six months from DOD.