I'm at the airport almost weekly, and see AS using C, D, or N almost exclusively for departures. However, there are occasional departures from A and even S. My guess is that these A or S departures happen when the inbound aircraft was an international arrival. Instead of repositioning the aircraft, AS uses the A or S gate for the next departure.

It's been long time since I've noticed AS using the B gates (it was often B9), but I go out there very rarely.

A few years ago, we got to SEA and saw our flight to BOS was set for an S gate. We went to the D lounge, figuring we'd be well-positioned to take the train to the S gates if necessary. Not long after we got to the D lounge, our flight was changed to a D gate.