My thought:
There may be impact on dummy fill on interconnect in spice extraction as it depends on which technology you are in. For older node technologies I do not think there would be any impact on having dummy metal fills in design or it can be minimal. However in case of latest technology nodes, there is impact on having dummy fills near to critical nets.
Also in case extraction deck consider dummy fills, then there is point on the running the design with having dummy metal fill shapes to see if . In case extraction runset does not consider the dummy fill then there is no point in running the design with having fill shapes.
On EM, I do not think there would be any impact on dummy fill as EM is current density flowing through metal shape.