Actually the correct answer depends on the propellor and the transmission. There was a lot of research done on this a few years back.
The pretty widely reported results came up that a two blade prop fixed in the vertical position has less drag than a two blade prop either freewheeling or fixed in any other position.
A three or more blade prop with minimal resistance on the shaft will have less drag free wheeling, BUT if there is much friction on the shaft, the turbulence of the semi-free wheeling propellor is greater than that of a fixed prop in the ''Mickey Mouse'' position, which is the lowest drag position for a fixed three bladed prop.
Then there is the issue of transmission wear. In all cases freewheeling causes more transmission wear than being fixed. On most small marine transmissions there is no problem with lubrication or heat build up. That said some larger transmissions have shaft driven lube and cooling pumps which are not being turned when freewheeling.