First, I am very new to leather work, but I see no need to finish something that is unseen, or in the case of a knife tang, counterproductive (possibly detrimental, could cause failure of the glued on scales due to the glue not bonding to the polished metal). Christopher Schwarz noted woodworker, author and teacher sees no point in obsessing over unseen parts of furniture.
2nd, I asked my cousin, retired New York State Trooper, currently a captain in a local PD (also their training officer and Glock armorer, so he is not an uniformed man), the question of using a signal blinker in a lane designated as turn only. He had asked many senior NYSP over the years, never got a consistant answer. His take and mine are you are required to signal your turn. You have signaled your turn, you are in a lane that requires you to turn, the lane signs and marking have signaled your intent. The warning of your intened action has been conveyed to all those around you.