My hiring date was 11/16/17. That indoc day new hires submit preferences for bases (SEA, LAX, PHX). This preference remains a standing bid.
The week I started ground school (12/4) the results came in and I got LAX, my second choice. The next vacancy bid results (to go into effect for March, I think) showed seven new hire FOs junior to me getting SEA. I also got SEA, and I haven’t finished training.
So I’d say if your start date is in first quarter 2018, you have a good chance of getting SEA out of training. Could change later this year, after all, this is the regionals.