As Racer said, YES in salt. I'll go even further - Always grease the prop shaft, salt or fresh water.
The old Merc in my sig was and is used exclusively in fresh water. When I removed the prop the first time, it was a bear. A little bit of grease would have helped way back when and doesn't hurt when I install/remove the prop today.
Another thing that is great to do is pressure wash the inside of the outboard as it gets rid of gunk and old lube that can work it's way into the electronics of the motor over time and help cause many of the electrical problems you see on this site. I do it before I work on outboards if they are gunked up. It makes for a clean repair with no dirty mistakes than can cause those motor gremlins later. Hit those Zerk fitting too. But not too much as you see on many outboards that make for a crappy looking machine and is not necessary. Many think that outboard maintenance is putting half a tube of grease on the Zerks and unnecessarily changing sparkplugs.