People have asked me if I use the Aperture mode, and I've always said I use the Auto mode. Frankly, I didn't know it was called the Aperture mode in the Leica manual. It's the red A on the shutter speed dial on top of the camera and for me it is Auto.
But it's led me to clarifying how I actually shoot. Because I don't shoot Aperture mode. And let me be very specific here, because I feel this is very important to grasp for any Leica shooter.
The fact is that Leica lenses are quite unique. They're developed and produced to perform optimum fully open. Meaning, a Leica 50mm Summilux-M ASPH f/1.4 is developed to perform optimum at f/1.4. It's at that aperture value that is needs the least light, has the most unique and pleasant bokeh (how the out-of-focus areas take shape), the most interesting and playful depth of field (or selective focus if you will: only a thin layer of the image is in 100% focus while the rest is blurred and unsharp; forming the sought after bokeh).
From lens test of the 70ies I remember that most lenses was tested as "this lens performs best sharpness at f/5.6" when it was a Nikon 50mm f/1.8 lens. It's true that if you read some of Erwin Putts splendid reviews he will tell how contrast and corner sharpness improves at f/2.8 or f/5.6 on some Leica lenses. For landscape that might be considered optimum but for portraits and atmosphere photos, which is what I do the most, the quality of the narrow sharpness along with the play with light and the bokeh is what makes my day.
Hence, I always use Leica lenses fully open. The only reason I could ever think of for stepping down to f/2.0 or f/5.6 would be if there is too much light. I'm so fortunate to live in a Scandinavia where it's dark most of the year, so I don't have that problem. Those who live in Quatar, California or India must use ND filters (and often do so) in order to stay with wide-open lenses while shooting in sunlight.
Another reason to stop it down would be to do for example a group portrait of three persons or more where you want to make sure they are all sharp. Though if you're not going to use the f/1.4, save some money and start with a f/2.0 or f/2.8 lens.