Inverts are much more sensitive to salinity levels than fish. Many people do FW dips before adding fish to their tank but I wouldn't recommend this with say a cleaner shrimp if you want to see it survive... So running a low salinity during your cycle wouldn't really help your fish and just make it harder on any invert hitchhikers in your live rock.
If you do a FW dip on fish, make sure to match temperature and PH as sudden changes in either of these can cause shock.