I think it depends on the fans and what you have them plugged into.
For example, the EK-Vardars I have on my radiators have a "zero RPM" mode - when the PWM signal drops below around 25-30% they will actually stop; not that you'd want to use them as case fans. But you can acheive the same with a custom fan curve, but that depends on the capabilities of the motherboard BIOS or controller the fans are connected to - it's no problem to have a curve starting at 0% on the Aqua Computer controllers, for example.
It's likely you will need to have independent control of each fan (or group of fans, depending on how many you have) as you will almost certainly need to have at least some airflow through your case even when the PC is mostly idle - that'll depend on your case and components. Then it's probably down to the noise profile of the fans you have whether having multiple fans running at their minimum speed is quieter than one perhaps slightly above minimum.
Some fans won't like low % PWM signals and need a boost to start running, which might defeat the point - you could end up with fans spinnng up and down which could be more annoying than just leaving them running at a low speed.
In short - yes it's possible, but whether it's worth it for your set up would require experimentation.
