I don't recommend coasting downhill. Most bikes have some partial clutch engagement when the clutch is locked-you'll notice that the locked clutch position is not at the maximum point of clutch engagement. To check if this is your situation, start your bike and lock out your clutch. Pick up the rear of the bike to lift the tire off the ground and rev the engine. If the tire spins ever so slightly and it probably will while locked, it means that you may increase wear or glaze the clutch friction pucks if you coast down hill with it in the locked position. Check to see if the wheel spins with the clutch fully depressed by hand (not locked) and you'll see that it usually doesn't.
Yes, this occurs with a well adjusted clutch. If you try to completely disengage the clutch in the locked position, odds it won't fully engage or possibly slip when it is released for a start. THe situation worsens if you use one of those extra long levers.
There is no harm done in coasting down hill with engine engaged. If you need to go slower while in gear, then a 48, 50, or 55 tooth sprocket is your best bet.