I cannot see any benefit in incident metering the sky. But if the sky is an important element of the overall composition e.g. dramatic light behind equally dramatic clouds, spot meter the lightest (but not pure white), the darkest (but not complete black), and weight these readings 2x or 3x, then proceed to the landscape; weight readings here too, to give the meter something to really chew on. Invariably the sky is a couple of stops brighter than the rest of the landscape, and this is the problem so many photographers have, often erroneously giving too much weight to the exposure of the sky at the expense of correctly exposing all other (landscape) elements. As a rule in my work with Velvia, I do not include plain sky (at any time) in my images; an inspidly coloured sky is best given a complete miss when and where it is possible to do so, or at least reduced so that it is not such a distraction.
The use of ND filters and spot metering must be subject to extensive personal experimentation in-situ; there are many, many variations and outcomes.