Originally Posted by
United757
I'd say 5AM
I agree. Just did this exact flight (YVR-SEA) in late April. I got to the counter about 5 am and was in the Board Room by about 5:40. This was a Saturday morning. Monday mornings are MUCH busier, in my experience. US departures are generally busiest in the early morning as most carriers have at least one departure in the 6:30-7:30 hour.
You pre-clear immigration, so that adds to the required time. The good news is no worries about missing your connection in SEA due to customs delays.