Subtotalling by hierarchy is an option in Grid based reports where a crossjoin has been used. Subtotalling can use any one of the Sum, Avg, Min or Max functions.
To add a Subtotal, right-click on one of the members of the hierarchy you would like to add the Subtotal to, and choose the function you require under the XLCubed > Apply > Show Subtotals menu:
To remove the Subtotal, select the XLCubed > Apply > Show Subtotals > None menu item.
Cube vs Excel mode
The default is Excel mode, this will use an Excel formula to calculate the totals - this would mean any calculated members will be totaled after the query is run and may not return the number you want (summing the average or percent numbers etc.).
Cube mode performs the sub-totaling as part of the query, such that (if the cube calculations support it) any "Average" or "Percent" calculations will remain consistent.
You can use the "Grid" ribbon to switch between the 2 modes: