Currently there is no way to set a conditional value as a default.
Example - GI chase screen using Opportunities as base and parameters for From date, To date and Owner. Easy enough to set From and To using things like @Month-1 and @Month+3 but when using @Me for the Owner, if the currently logged in user doesn't have a corresonding employee record you get an error when accessing the dashboard instead of applying a "null" filter and displaying all records. Taking the default value out of the dashboard fixes the problem, but then employee users get all records returned by default instead of just the ones they own.
What is needed is an ability to test the current value of the @me entity and adjust return value accordingly (e.g. =IsNull( @me,'') or =IIf(len(@me)>=1,@me,'')).
The workaround is obviously to create a dummy employee record for each affected user record, but this then creates other issues.