Self directed opt-outs are pretty easy at Seatac. I'm not Pre-check eligible yet, so I can't speak for what would happen if you don't get the beeps. But at all the checkpoints, you can pick your own line after document-check. And, since the security lines move slowly, you'll have plenty of time to see which lines lead to WTMDs. Usually at checkpoint 5 that's the far left lane, though occasionally it's blocked off. The good news is that all the checkpoints are within a minute or two's walk, and all connect to any gate.

Seatac is my home airport--I fly out of there two or three times a month. It's an easy SDOO almost all the time, which is just as well, since the TSOs tend to be among the most 'bark-and-bite' prone I have ever encountered.