Basic rider training, and defensive techniques, dictate that the rider always be riding under power. Since it's impossible to be under power while the clutch is disengaged for a shift, riders are taught/coached not to shift in higher risk areas like intersections or railway tracks, or areas where the re-engagement of the clutch could cause problems if not done smoothly, like curves and turns. I'm reasonably certain this pretty standard teaching, historically, for any class of vehicle with a manual transmission. From a safety perspective, once you're comfortable with your shifting technique, you're going to do it in curves or turns if you need to, knowing that you have to be smoother. If you must shift in an intersection, you do it quickly to reduce the amount of time you're not under power.
As far as coasting down a hill - I'm not sure how often you would be doing that, but that could cause an issue when you let the clutch back out again if it's at any reasonable speed.
I have coasted - in very heavy (stop and go) traffic when my left hand gets tired. Click it into neutral and left gravity get me down the hill. Traffic is already bunched up, there's nowhere to go, and I'm at first-gear speed so I do run into trouble, I can quickly stomp on the shifter and get into gear.
