So for the first time, I'm going to be needing to fly somewhere along with my trombone. It'll be too big to fit in the carry-on bin, so my options are check it or buy a seat; from horror stories I've heard from friends, I'm leaning towards buying a seat.

Southwest requires that an instrument in its own seat be placed in the first row of the airplane farther from the aisle from any other passenger (so, ideally, window seat); does anyone have any experience regarding what I need to do to ensure that an appropriate seat is actually available when I get on to put it in, or do I just have to take my chances and, if nothing's open, gate-check it?