Keeping the skin on the fish helps hold its shape during cooking. You can remove the skin after poaching, as it will be softened and not all that texturally appealing. It’s easiest to peel off the skin once the fish has cooled to room temp or chilled. If you have fillets that do not have the skin, that’s fine, just use care (and a large spatula!) when removing the fish from the pan. That's especially true if you want the filets to stay whole. If you are using the fish in an Herbed Salmon Salad, for instance, or another dish where the salmon will be chopped or flaked, this matters not a bit.