Good question. Part 135 turbine time is never a waste and still helps the "fast track" process to the majors. It's also good experience if you ever decide to go corporate or charter in case the airline gigs don't work out too well. The main thing everybody looks at now besides turbine time is crew experience. Flying single pilot freight in turboprops is great for building time for a couple of years, but you'll need the crew time soon. Regionals are probably the best place to be right now if you want to get to the majors and many are dropping the turbine PIC requirement. However, if you want to rack up a thousand or so PIC hours to check off that box on your apps, flying single pilot freight for a year or two will be the fastest. No waiting around to upgrade.