It depends on the recipe! If you are cooking the fish in a pan, you may need to flip it for even cooking. Another option is to cover the pan for part of the cooking to trap the heat and allow the top to cook through before the bottom overcooks. If you are transferring the pan to the oven, then, no, you do not need to flip the salmon. And if you are cooking fish in the oven, then no need to flip in most cases.