Neither! For roughly level ground I will have the length set so that my forearm slopes down slightly from the elbow. I walk often enough in the rain, and this ensures that when I am wearing a rain jacket, rain does not collect in the sleeve and pool in the elbow of the jacket. Of course, if I were truly well disciplined, I could always shake the water out before lifting my arm for some reason, but I'm not. You can imagine the thrill of remembering too late, about the time a stream of water is running down inside the jacket.
From that initial position, I will adjust the length when going up or down steeper slopes. I find this more important going downhill, where I will extend the pole length by as much as 20 cm for really steep slopes. My aim is to ensure that I am still walking upright, and not leaning forward to plant the pole tips well ahead of my feet going downhill. It is possible to maintain a normal or slightly faster pace going downhill doing this while using the poles to maintain balance and stability. If the track surface is even and doesn't have too many large rocks or other obstacles, it is possible to gain considerable speed on steeper slopes doing this while still maintaining good control.
Less often, I will shorten the poles going uphill, but not as regularly as I would going downhill. On less steep slopes, I find it just as easy to lift my hands up a little and plant the pole tip a little closer to the heel of my leading foot than the toe. Only if the slope gets steeper might I consider shortening the poles.
Over the years on the camino, I can only recall a couple of times where I have found it has been necessary to shorten my poles, but I have regularly lengthened them. What I don't ever recall doing on the camino in Spain and Portugal is differentially lengthening or shortening the pole on one side or the other to account for steeper side slopes. I have done that just once on a pilgrimage walk, in Norway. It was on a short section of the Gudbrandsdalsleden shortly after Kongsvoll. Those who have walked there might recall the quite savage sideslope for the first few kilometres where the walking track is further up the valley wall to avoid the E6.