To float or to double clutch, it is always better for the equipment to use the clutch, it just relieves the pressure, keeps you from dropping the drive shaft or worse. So when in doubt use the clutch. Just a better habit.
Because it is the correct way to shift a non sync trans. New drivers seem to have the mis-conception as to it importance. I was given a driver this week to train to haul liquid in a non baffled powder train, he's been driving for our company for 4 years, and I had to teach first how to shift correctly. Believe me it matters with a smooth bore tanker. Also the wear inside the trans is much greater to the gear faces when driver do not know how to shift correctly. If you think you can shift hold your hold coffee no lid in your left hand when your shift. If you burn your hand you do not know how to shift.