If the horizontal is skewed by up to 2 or 3 degrees it won't matter enough to notice. But it would look terrible. You may want to just adjust it for a bit of skew but not too much up to the point that the misalignment isn't too noticable.

The fin mounting should be altered to sit parallel to the wing's center line. Or if this is a profile fuselage then I'd be worried about the effect of the added "camber" of the curved fuselage on the knife edge or just for general flying. If it's a built up fuselage the effect won't be as noticable but it would still be there.