There is another way to get a B&W photo effect. In my opinion this is what should be most close to original black and white photographs.
Before color photos, cameras registered only the amount of light that entered the lens, not the color. If you desaturate a color image in Blender (through a HSV Node), you take the highest value of RGB and make all channels that value (RGB = 0.8, 0.5, 0.1 would result in RGB = 0.8, 0.8, 0.8).
the RGB to B&W Node calculates the Luminance (average human perceived brightness).
But an old black and white camera would only see the amount of light, not what color it was.
So, from top to botom, from useless in almost any situation, via a visual satisfying solution, to (I think) most accurate:
So a simple node setup like this:
Would do the trick.
Of course, feel free to use whatever you like most, artistically, but if you want to get close to B&W photography, at least try this.