From Airworks: By the way, WD40 is non-volatile when dry. The main active ingredient is a viscous oil that remains behind after a hydrocarbon (providing the fluid factor) evaporates. The remaining oil provides lubrication and moisture protection.

Using plain fresh water is fine for removing obvious dirt and grime usually present in small quantities even after a freshwater dive. The minimal moisture that can remain after proper flushing can be mitigated by using a light amount of WD40 sprayed onto a Qtip and applied carefully to moving metal parts of the 2nd stage. No worries; it will improve the life of your reg.