Is the WDT disabled by default in the 328P Arduinos?

I've been doing some work on the Power Down sleep mode, and find that disabling the WDT doesn't change the sleep current, which I assume means it's already off. Also, I've never bothered to disable or reset the WDT in my software, and so far as I know it's never reset me.

Assuming the stock fuse settings and bootloaders for parts like the Uno, Nano, Pro Mini, and the breadboard PU chip, is the WDT off by default?

You could turn on the watchdog timer and see if the current consumption goes up. The WDT is off by default, for any of the Arduinos I've used. But there are many suppliers and many bootloaders.

The fuse option WDTON is by factory default unprogrammed on AVR based Arduinos.