Currently, you can only sort group sections in Report Designer by the fields that are actually used in the group section. And you get two options: Ascending and Descending (click here for screenshot).
It would be nice to have a field called SortExpression in the above screenshot with a ... button where you could define a formula expression.
Then, for example, on the Customer Summary report, you could set SortExpression to "=Sum([ARBalances.CurrentBal])" and SortOrder to "Descending" to get the report to sort in descending order on the Current Balance column.
It would also be nice to have the ... button available in the Enter sorting conditions here section on the Sorting And Grouping tab of the File -> Build schema... window in Report Designer. Then you could sort the detailSection of the report by a formula expression in addition to being able to sort the group sections.