Soaking dilutes some of the potato's natural moisture, and allows the cook to remove a lot more moisture than (s)he would otherwise be able to. Dilution aids the cook in controlling the amount of starch, and reduces the bitterness in some types of potatoes.
Mashed: Not adverse, but waste of time. Just peel, cut into more efficiently sized pieces, and boil in well-seasoned water. Control the texture of your mashed by type of potato chose, and mashing method. I.e., ricer, food-mill, masher, etc. Riced (PITA) russets will net maximum fluffiness; smashed are a close second, and milled a very close third. I prefer smashed for the texture, as do most adults. A very common sin with smashing is overworking the spuds. Brings out the starch and makes 'em gummy. Another common sin is too little liquid. Never, never "whip" them with a mixer or whisk.
Roasted: Peel and soak for a little while. Overnight is not necessary. Dry the surface well, or the potato won't brown as nicely. A little oil is nice.
Sauteed: Most def'. Do just what you did. Glad you asked. You da bomb, girl.
Gratins: Hmmm. So many gratins, can't give a generic answer. If you relied on the natural potato starch to thicken the cream, you certainly wouldn't want to limit the surface starch. On the other hand, if you're mostly looking for a crisp, brown top -- then you would. Also depends somewhat on the potatoes chosen.