If you replace those cylinders, the new ones will not have any fluid in them, so yes they must be filled and bled. Do you have the new parts? Are you doing this yourself? The bleeder screw should be evident on the slave cylinder. I would also recommend changing the master->slave hose if you're doing both of those, and the SS braided aftermarket line is actually cheaper than OEM and obviously a lot more durable.
If you are going to lose that much fluid anyways, it may be an idea to drain and re-fill your entire brake system If you haven't recently. A good amount of people use ATE super blue/gold and are very happy with it (myself included).