If you're using a spinning reel, I'd probably go with 10#... give or take 2# either way. My last chinook (low water springer on the Clack, about a month ago) was on 8# -- certainly doesn't give you much pull, but light line hooks more fish (don't ever let anyone tell you different). It's better to have hooked and lost than never hooked at all.
I'd also recommend leaving your drag just a hair loose, but still tight enough to get a good hookset. You can (GENTLY) use your forefinger to slow the drag up, but it's hard to go the other way. Drags that are too tight lose more fish than drags that are too loose. Also, if you're using anything but top-of-the-line spinning reels, a couple of "hot" chinook will be the end of your drag washers... your fingertip heals quickly.
