The "ideal" moisture content (MC) depends upon the use of the wood and the annual average relative humidity at the place where the wood used. It is critical that the wood you work with is dry down to an MC within 2 percentage points of the equilibrium moisture content (EMC) of the in-use location. Whether wood will be used indoors or outside also determines how dry wood should be.
Wood will seek equilibrium moisture content (EMC) in relation to the relative humidity (RH) and temperature of its surroundings. Wood species, density, open, closed grain also factor into how soon wood dries. Weighing blanks probably the best way to tell if pen blank has reached EMC
If you dip a pen blank into wax or tar sealing all four sides, this will stop moisture transfer. Wood will always gain and lose MC throughout useful life due to relative humidity. Finish will slow down this gain or lose of moisture but not stop it.
So when you buy pen blanks completely sealed in wax, scrap off wax from sides and let blank hang out couple of weeks or months before use. If buy pen blanks not sealed let them hang out couple of weeks before turning. If cutting your own blanks cut slightly over sized and seal ends with wax or end sealer again let hang out for couple of months.