I read this article before making our first reservation flying with a cello which was quite helpful. Our booking is with Southwest which does not pre-assign seats so I was instructed to request pre-boarding as you wrote. One difference was that the reservation agent booked the seat under my daughter’s name and assigned it as an extra seat for her. Like you would do if you were a large person that needed two seats. This is better for us since the seats are not pre-assigned, so at least she is assured two seats next to each other. (Well I use the term assured lightly!) Anyway, this seemed like a good idea so thought I would suggest it.