Why did Joseph tell his brothers not to tell Pharaoh they were shepherds?
Joseph instructed his brothers not to tell Pharaoh they were shepherds because a shepherd was an abomination in the eyes of the Egyptians. He knew that their occupation would be offensive to Pharaoh and the Egyptian culture, so he advised them to avoid mentioning it when they settled in Goshen.
Les explains that Joseph made it plain to his brethren when they came down to Goshen: "for goodness sake, don't tell Pharaoh that you're a shepherd of sheep, because a shepherd in the eyes of an Egyptian was an abomination." The Egyptian culture regarded shepherds as an abomination, so Joseph warned his brothers to avoid that identification before Pharaoh. This same principle of being considered an abomination appears in the context with Moses and Paul, but Joseph's immediate concern was the Egyptian view of shepherds.