When a Dashboard is published to a server, the parameters associated with the dashboard are set to what they were when published.
You can then change them when using the dashboard, but they aren't saved at that point. ie the next person who uses the dashboard will have parameters set to those when the dashboard was published.
If you want to allow these parameters to be changed (by managers) then the workbook will have to be re-saved (or republished)
An option to do this would be to allow managers to have WebEdit permissions to the workbook, where they can change the parameters, and then save the workbook.
You could Have a workbook with the dashboard as a published View, and a sheet with the parameters on them, but not as a published view.
ie. The end user sees the published dashboard, where parameters have been set. The Manager is able to webedit the workbook, and has access to a Sheet (which isn't a published view), and can amend the parameters, and then save the workbook.
The option would work, but im not sure how happy i would be with allowing someone to overwrite the published dashboard.
Hope this helps