I don't think anyone has explicitly said that if you have a gear trans you are not expected to throttle up and down while switching between fwd and reverse, especially without foot throttle. This is a separate issue entirely from lowering throttle to engage the pto, which is a good practice on most tractors which are fancy than yours but likely not worth the thought and effort on yours between shifts, or at all, especially if you have the two stage clutch. If you do have two stage it would be beneficial to learn to use it, but I wouldn't consider it detrimental if you slip up occasionally. Sometimes its more important to just get stopped than worry about negligible wear on the clutch. Like when you are heading into your house, pond or getting ready to get taken off the tractor by a branch.