If you're coasting because you're uncomfortable, like your butt or hands, then fix that issue. If you're not comfortable at the speed, address that with practice or better tires or even bike geo. If you're tired and using coasting to recover, back off the pace so you can put out more consistent power. If it's terrain maybe it's ok.
Personally I prefer to soft pedal rather than coast on long easy descents, more to keep limber than for speed.