Don't worry about the 6 am flight time, it'll still be pretty busy at Seatac at that hour. Give yourself a few minutes to check out the security checkpoints--other than the northern most one, all of the lines of the others are fully visible before you get into the line for document check. (The north checkpoint's far right lane is usually WTMD only, but I don't use it unless I can't find a WTMD open lane at the other checkpoints, since you can't actually see if it'll be safe before you get in line and commit to that checkpoint.)

The others are easy to scope out first. If you feel self conscious, just put a cellphone to your ear and pretend you're on the phone as you stroll by the checkpoints and check them out. No problem... After the document check, you are free to pick your own security line, so pick the one with the WTMD and you're home free.