I think late Feb to the first week in March. Watch out for Seattle and Vancouver spring break in March. Vancouver gets 2 weeks! I didn't realize that until this past season when I was there on a Monday, ready to be all by myself in line...ha! It was the 2nd week of Vancouver's break and lines were loooong. But Feb and March are busy in general. If you are very crowd averse, can you do early January?
If you live in Bellingham, why would you book in advance?
Because places book up. And if he wants something in particular--a great value, slopeside, a particular condo complex, whatever--you do better by advance booking. I have to book well in advance to get the dogs-allowed places that I like. Sometimes we luck out at the last minute for weekend trips but for a week, I wouldn't leave it to chance.