Since all of your grounds should be connected into a single net by traces you should only need to set the seed on a single point and everything that is connected to that net will be part of the ground fill.
The need for extra vias is on a board to board basis. For example you may have an area of fill that does not actuall connect to the ground net because it is surrounded by other nets. In that case you can add a via to connect it to the ground fill on the other side. If the area is small enough you can even delete the small section of ground fill. But this need for them to be all connected depends on what type of circuit you have and how much noise there may be. In most cases unless you are designing high speed communications circuits you do not need to worry to much.
The other reason for adding vias is to move heat from one side of the board to the other. So if you have a voltage regulator or FETs that are dissipating a lot of heat you can add lots of vias to move the heat to the other side of the board. There are many more things that you would want to keep cool those two are just some common examples. If you unsure if a component needs extra cooling check the datasheet for the thermal recommendations.