Any spare slots put earth stones in for the simple reason that as stated many players put earth in their weapon.
Personally have a set of amour for all situations to make sure that if a player asks for PvP you can resist to the maximum 80% without fear of loosing any stats you have Star Wars Galaxies credits.
Earth are the best to put in amour so you can defend more as well as reduce other sores and zeros attacks, magic stones for archers and hunters imp as they usually put these stones in to boost some of their attacks. The others mainly go for earth or fire so stick with them.