One thing to be careful of on Alaska/Horizon is if their JAX (or however you spell the acronym-which is their tie in to CASS) is down they wont even let you sit in the back if you aren't approved for the cockpit. Most airlines will still let you sit in an empty seat if you come back denied for the cockpit, but there are a select few who wont....
That said, Alaska and Horizon are always great to jumpseat on. They have a good group of people there! I can tell you that Friday the 30th and Saturday the 1st are just SLAMMED busy for them....and as HP pilot said, traffic between PDX and SEA can be nuts!