There is one specific reason you should always use a hood with this lens. The ZM 50 Planar is one of the most flare resistant rangefinder 50s you can use. But it does flare, often badly, if light shines directly on the stupid chromed bayonet for the hood and then reflects back onto the lens. So I have a tapered plastic square hood from tahusa.co, which does a good job of keeping light off the *&^%ing bayonet. Cosina and Zeiss, please make the bayonet matte and black.
Marty*
*I am a Zeissaholic, even if Cosina makes them. Tomioka made many of the Zeiss Contax SLR lenses (AEJ and MMJ) after all.