Ignorance is bliss

There is a certain point, usually about FL280, when speed control is changed over from indicated, to mach speed. That would be I believe the point, where Vmo stops being limiting, and Mmo takes over. For practical reasons, at this area airplane will start to control your speed in climb or descend VNAV regimes and also cruise in Mach numbers.You might for example see a 278/.775 climb on CLB PROG page.It is practical then, although I believe not required, to plan speeds in Mach in Flightplan.