native ISO sensitivity (which typically offers the highest image quality)

baneling wrote:

set to its native ISO sensitivity (which typically offers the highest image quality).

There's quality, and then there's quality. And quality, and quality. There are a zillion different things that contribute to IQ (image quality), and using the "native ISO sensitivity" trades off depth of field and image sharpness in order to get a reduced noise level.

When you're trying to get the best image quality in a landscape photo, the ISO setting should be the last thing you'll be concerned about in the exposure triangle. First is the aperture. Pick an f-number high enough to get the depth of field that you need, but no higher. Then pick a shutter speed sufficient to deal with camera shake (if you're not using a tripod or Image Stabilization) and subject motion. Yes, landscapes move, unless there's absolutely no wind at all. Personally, I like 1/200 or so as a starting point.

Once you've got the aperture and shutter speed selected, then pick the ISO that will let you use that combination. Note: you'll probably find that if you're using a polarizing filter, you'll need to use a higher ISO than you'd like. In a Sunny-16 situation, getting 1/200 at f/11 is going to call for ISO 100 without a polarizer and ISO 400 with a polarizer.

Although I own a very nice, very pricy polarizing filter, I almost never use it on landscape photos. I'd rather have increased shutter speed, higher f-stop, and/or lower ISO. I can get nice blue skies simply by not over-exposing — I usually have about -2/3 Ev of Exposure Compensation dialed in on my XT/350D.

[A side note that's not really relevant to this particular situation: I also use ISO 200 as my minimum ISO when shooting Raw, because the XT/350D has limited highlight headroom at ISO 100. This doesn't affect in-camera JPEG, and as far as I know, it doesn't apply to any later "Rebel" models.]