[Feral]

You're generally pretty safe dropping used quest items. However, to be on the safe side, you might want to find a place to stash your items (the same applies to surplus gear). Most items will disappear (find new owners) if you leave them lying around. IIRC, this doesn't apply to critical quest items. But, to avoid problems, find a container in an empty building and stash your stuff there. There are many throughout the city. One caveat - don't choose a place that is a future/potential stronghold area because you stand the chance of losing the items for good if/when the area becomes your stronghold.

By the way, if you have completed the Temple Ruins quest, then you used the Symbol of Amuanator without knowing it (which often happens with other keys as well).