Surprisingly popular thread:
1. M2 Yes. I have also heard that the shutter is under little more tension cocked. However, Gus Lazzari who services Leicas, advises against keeping it cocked as he has seen curtains tearing away from their anchor points. How he is sure this is linked to precocking and not storing it for months cocked or other abuse I am not sure, but he is very knowledgeable. I usually don't put the camera away cocked, but I am happy to do so for the few days till the next use.
2. M6. Yes, but not if it's going into my bag where firing the shutter is nothing compared to continual activation of the meter and draining the battery.
3. M5. Yes, out of habit, but I try not to because the meter circuit is active till the shutter is fired. I remember my science teacher would regularly waste a frame in his M5 for this reason. In practice, apparently, the drain on the battery is minimal, and if I put it away with a lens cap on no harm done. The beauty of the M5 is that you can tell if it's cocked by feel as a light pressure on the shutter and release will cause the meter arm to retract and return to the metering position with a palpable vibration.