You sound like you are considering using the filter sort of "by default", but you don't want to. You only want to use it when it produces an effect you want.
With the 10-20mm Sigma, my experience is that you don't have to worry about the filter cutting off the corners of the image, but you should check with your filter just to be sure. it's not fatal even if it does, but just something to keep in mind.
You do have to be careful with clear blue skies, particularly with a lens this wide. You'll get some unevenness anyway, just due to discrepancies in the lighting across such a wide area of sky.
The filter will cause light fall-off to varying degrees, depending on how you rotate the filter. The light loss can be handy (if you don't have an ND filter), or not (makes focusing difficult/impossible, and makes handholding more difficult.)
You'll see the polarizing effect as you rotate the filter, but you don't always want the maximum effect. Sometimes just a moderate effect is best, because full polarizing can make some subjects looks somewhat dull and flat.
Any filter will compromise image quality, and while I have only very inexpensive filters for this lens myself (it's possible to spend as much for a generous set of filters as this lens cost), you do want to be conscious of flare, and consider not using the filter when there is a significant light source in the image itself. Usually I find that a polarizer isn't that helpful when photographing into a light source anyway, but that suggestion would apply to any other inexpensive filter you might use as well.
Paul
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