There has been a recent post by someone who supposdely got the word dorectly from from Yanmar that reccomends leaving the transmission
in neutral. You might want to google
it and see what the reasoning is. I always put my 3gm30
in reverse because I can't stand the noise
of the prop turning. Some transmissions can be ruined by allowing the prop to rotate.
Depending on your boat type and prop, there may be significant reduction in drag by locking the prop in position. At the worst, it's only a limited benefit to let the prop spin. There is no thrust from a prop spinning from hydrodynamic pressure, just a possible limited reduction in drag that mainly diehard racer's would care about.
In my own experience sailing identical full keel
boats, saw no difference in speed fixed or spinning. Three identical boats on 3 plus day ocean passages in ideal reaching conditions, one with a 2 bladed prop fixed in the shadow of the deadwood, one with a spinning 3 bladed prop, and one with a fixed three blade. The differential in daily mileages covered was less than one percent. Obviously these weren't side by side tests but the conditions were as identical as it's possible to get. Average daily speeds were slightly better than the theoretical hull
speeds of the boats, irregardless of the prop or whether it was rotating.