They all vary. All do some form of pre boarding for first class, frequent flyers and those with children. Many have some sort of premium coach, such as the very first row in coach boards before the rest.
Virgin America has followed Southwest in using groups, rather then seat assignments. The others assign seats and board by zones. Alaska boards the main cabin in just two zones - row 15 and above, then all rows.
I find this interesting because, while employed by Boeing, I was in a team that built a simulation as part of a process to analyze minimizing the turnaround time of passenger aircraft. Passenger boarding is a big part of that process, but we looked at everything - fueling, cabin cleaning, loading food and water - everything. With the data we had from a major airline, we found that boarding by zones slowed the process. It was best after pre boarding to call all rows. Alaska Airlines did that for a few years recently. According to Cook they now break into two zones, instead.
Airline Reporter reporter Colin Cook in Seattle, WA.