Do you run the AC all the way until you turn your car off? I have heard that this can leave moisture in the system and cause that musty smell. When I run my AC, I tend to turn it off a mile or so with only the fan on before I park to reduce residual moisture in the vents. It may be a placebo effect, but it made sense to me.
Ive heard that before as well. I turn it off a few minutes before I get home and just let air blow and still the same issue.
The ac on my wifes BMW is always on and never had an issue.