A hybrid system like Delta would be beneficial. It triages the JS by importance. Can book 5 days out to go to work on a first come first serve, and something like 3 days for returning from work. personal travel is something like 2 days prior.
Straight check-in time doesn't protect the real intent of getting to work, ant pure seniority doesn't mitigate the pitfalls of not knowing you really have the JS until 15 prior.

So, instead of all or none, AA being check-in time and UAL straight seniority, the hybrid Delta style has my vote.