Here is a little trick:
When apping to a guild as a caster or healer, put some wizard oil on your weapon before logging out.
Armory will include the added +spell damage in your numerical stats, but the wizard oil itself will not be visible on the weapon in armory.
This is in contrast to elixirs, flasks, and other buffs were the person viewing your profile will see the buffs and realize your +spell damage is actually lower. The only way this can be detected is if the person actually bothers to add up all the stats from all your items using a calculator.