Does it make a difference whether the moka lid is open or not? When brewing coffee with a moka pot, I often leave the lid open so I can better tell when the coffee is ready. Is having the lid off affecting the brewing process in any significant way?

I imagine leaving the lid on will cause the water to heat up faster as the hot air inside the chamber cannot escape as easily, and I wonder if this affects the flavour or general coffee experience.
So what I've been taught is that you should keep the lid open while brewing. This is so you can watch the progress of the extraction, and cut the heat at the appropriate time. It gives you much more control over your brew this way - for example, I cut the heat (and run the base under the cold tap for larger pots) as soon as I see the coffee running from the spout turn lighter, rather than waiting for the gurgle.

However, as for lid open/closed during brewing, I don't imagine this has any effect on the extraction. The amount of heat lost will be tiny in comparison to other variables.