From what I hear tell, SWA has it pretty good in this regard. Maybe cause all their trips seem to be 3 days it makes it easier to plug and play, dunno.
The last part is true. 3-days are the easiest currency to work with in our trip-trading systems and are the majority of what scheduling builds. One of the recent articles from SWAPA had the 2018 breakdown as 9.5% turns, 11.5% 2-days, 64.9% 3-days and 14.1% 4-days (rounded). The 3-day number is actually down a bit, but the 1- and 2-day numbers are up. I've found the 4-day trips to be the hardest to manipulate when tweaking my schedule.
However, completely dropping trips (you giving back to the company) is impossible except for mil leave, and vacation drops. Giving away trips (someone else taking a trip from you) is a bit easier, but is highly dependent on the quality of the schedule you were able to bid (weekdays, 3-days, start time, commutability, etc). A lot of guys try to maximize their schedules by "clearing their board" and picking up stuff that pays better and/or playing the premium game - which hasn't been too great lately. But again, success with that is highly dependent on seniority.
Just my perspective three years in and coming up on 50% seniority in seat/base.