I also use the Independent Sector's hourly wage, I do send this to my Senior Management. I use the Volgistics volunteer software and when I enter the value it multiplies it by the hours and calculates the total savings and equivalent of FTE's.
i have always used the Independent sector figure.
I report this when I would do my annual budget presentation to Senior Management.
How do you calculate the hourly value of your volunteers? Do you use the info published by Independent Sector? Other? Is this part of your reporting to your senior leadership?