Portraiture is an art form and there are no rules in art, you are free to follow your heart. That being said, if you are doing portraits for a living, your goal should be pleasing your client. Likely what your client wants is an image that is a close match to what he/she sees in the dressing mirror. In other words, an image that approximates the dressing mirror perspective.
How this relates to focal length: Many think, 105mm is idyllic portrait focal length. This comes from the following:
The typical commercial portrait will likely be an 8×10 inch image displayed on a mantelpiece. Likely this image will be viewed from about 3 feet distance. The dressing mirror perspective is achieved when the viewing distance is approximately the focal length multiplied by magnification (enlargement). To make an 8×10 from a full frame requires approximately 8 ½ x magnification. Suppose a 105mm lens is used. The viewing distance becomes 8.5 x 105 = 893mm ÷ 25.4 = 35 inches.
This is the origin of this rule-of-thumb, it is not engraved in stone, follow your heart.
As to landscape, most choose a wide-angle 28mm thru 35mm.
