(Reuters) - Both advocates and opponents of legalized marijuana reacted with caution on Saturday to signs from the White House that growers in U.S. states where the drug is permitted would be shielded from federal prosecution, saying it was too early to know the final impact. FILE PHOTO - Marijuana flags are seen as protesters gather to smoke marijuana on steps of the U.S. Capitol to tell Congress to "De-schedule Cannabis Now" in Washington, U.S. April 24, 2017. REUTERSYuri Gripas U.S.