Hello fellow Mike P
You can always setup a dashboard with a sheet swap using the parameters you setup.
First, duplicate your sheet, turning off subtotals for one view. Then, follow the below link to setup the sheet swap:
The user will then be able to pick the view from the dropdown using the parameter and the view will show or not show subtitles based on the parameter selection.
As of right now (and someone correct me if I'm wrong), I don't believe there is a way to have dynamic subtotals otherwise.