Do most DSLR users keep the ISO setting on auto or a specific setting outdoors.?

To me auto is a bad idea. What you're doing is allowing your fully controllable camera to make decisions for you. That's now these cameras were designed... they are fully controllable.

So, unless a certain situation dictates something different I manually set my ISO at 100 and leave it there. Get into a darker situation, don't want flash, boost the ISO then.

Keeping the ISO at 100 will always insure you'll get the least noise possible on any particular shot.

After spending most of my life with film cameras, I like ISO 200 over ISO 100. It is a natural transition to almost know the f/stop and shutter speed, when outdoors looking at the light. I usually use a fixed f/stop(aperture priority) but don't do it all the time. On film cameras I have had the lenses tested for optimum lens opening. Since I am retired and my work is not that critical I just use a small f/stop and let the camera choose the shutter speed (shutter priority)..

Distortion? I have not encountered distortion.

I want to know how much noise I will be encountering after my shoots.

I set my ISO manually, my white balance manually and use the camera in the manual mode, using the cameras light meter to determine the proper exposure.