In a previous career era of my life I used to travel to little advanced networking tech conferences of CEOs and bankers and such, all in suits. I was working in Silicon Valley on supercomputer networking. They were using the same tech to connect massive arrays of disk drives to financial tracking systems. Modern fiber tech has since surpassed the tech involved, but it was very exciting at the time. I was the sole "tech" in the room besides the company reps. I wore network tech company T-shirts I had collected from friends. I also wore a black leather top hat. I stood out like a sore thumb, but a lot of the execs visibly envied my comfort level.
One time, the president of the conference tried to out-do me in casual by declaring the opening night meet and greet as t-shirt casual only. I showed up in a black sweat shirt made for me once, artfully appliqued to look like a tux, complete with faux pearl buttons that didn't button anything. It went with the leather top hat like a charm. The president guy thumped my on the arm in admiration and annoyance, with more force than necessary, but we all gave up on competing on casual and I continued to be me.
The point is dress to be comfortable. Seattle sounds like a place where sweatshirts would be comfortable. Geek-wear is always a good "I'm a rebel, too." fashion statement. Besides, you can't give your best demo if you aren't comfortable, and a lot of potential customers pick up on when you aren't comfortable. Some of the suits I met didn't know anything about the tech they supervised, but they knew almost everything about the company reps and their presentations. I was always glad I wasn't the one selling expensive and experimental network hardware to those folk.
Good luck on the demos and everything else.