If you have status with BA, AA, US (and others), or work for a large company or buy a flexible ticket you should get seat selection for free. If you don't, and it matters, then you can pay for seat selection after your ticket has been issued. At OLCI, when leaving the USA, there's a good chance of getting an aisle or window seat in WTP at OLCI when it opens, it's not so popular to do it from that direction on the dot of T-24. But before that, you can check Expertflyer (or use the experts here) to find out how things are going. No point asking until about 10 days before travel. WTP is also the best place to get an upgrade and SEA is extra good for that too! So you may want to get an AA, US or BA frequent flyer number into your booking. No guarantees however.