To be honest, I've never caught whitefish in the winter (when they are supposed to be most active), I usually catch them while trout fishing. I have found them in the Sanpoil River, Green River, and the Yakima River. The Green, Carbon, the Yakima, and the forks of the Snoqualimmie are the premier winter fisheries. I have heard of whitefish being caught in the Queets and tribs, as well. They take nymphs of all sizes, but on the Yak I have found them rising to BWOs.
Whitefish live in the slower water, not the frog water like SRC, but places like the deep slots below a riffle the long slow sections of rivers, places you don't normally find trout.
They hit nymphs very well, I landed a whopper whitefish in the upper canyon stretch of the Yak just above Thorp last month on a black stonefly nymph. If you are fishing nymphs you generally need to make a presentation right in there face as I have not often found them to move far out of the way to take them. I have often caught whitefish on drys, but typically only when there is a hatch. They can be a little finicky and leader shy. Comparaduns (and tiny any patterns in the fall) work well.
They are common and prolific in most local rivers. I sometimes fish for them when everything else is slow. I think they are the most fun to fish for when you can sight fish for them.