With the "newer" transmissions putting them in reverse is not a good idea. Over an extended period the prop will wear the clutch by the shaft being driven backward into the mechanism without the rest of the transmission "working" as would normally occur: HOWEVER,

In addition, one shouldn't let the prop free wheel because it will negatively affect the bearings, etc. because the tansmission isn't getting its normal lubing and cooling from the action of running -- or you're artificially wearing the transmission (at best).: SO,

Either i) use a shaft locking mechanism; ii) get a folding or feathering prop [momentarily stop it then return to "neutral"; or iii) live with the consequences [whatever they might be].

That's from Mack Boring's Service Department -- East Coast disbtributor for Yanmar (908) 964-0700.