We did a Sun Alaska cruise in August of 2007. They did say no jeans in the dining rooms at dinner time and I have to say that I don't think people did wear jeans at that time of day. Their "formal" nights were "dress up or not" so while some were in suits and dresses, others were just in their collared shirts and dress pants as they were the other nights.
Welcome to Cruisechat. I'm sure others who may have been on the Alaska cruise more recently will let you know if things have changed.
It's an absolutely teriffic cruise and you'll have such a wonderful time. That's a long way to come - From Scotland. I hope you'll be spending some time in Vancouver before or after your cruise because it's a beautiful place to see as well.