If you're in coach, be sure that the lap infant is in the reservation as soon as possible. If there's an empty seat next to the passenger with the lap infant, AS blocks it. They'll still assign it at the gate if they need it, but it will be (one of) the last seats they assign if the flight isn't full. When we had a lap infant (in 2014-2016), we wound up with the empty seat about 50% of the time on Alaska, including on flights where that was the only open seat in coach.
AS was by far the friendliest of the airlines we flew with a lap infant (which included AA, US, BA, QF, VA, NZ, and DL), both in policy and the helpfulness and friendliness of check-in agents, gate agents, phone reservation agents, Twitter agents, and flight attendants. It led to us booking with AS over the competition on several occasions even when they were a bit more expensive or less convenient.