Pretty much you can dump whatever you get from one dungeon after you've used it. The most notable exceptions are:
Signet of the Mavin (obtained in Crypt, needed at the Oracle Caves)
Masque of Evil (obtained in Serpent Temple, needed at Castle Shurugeon)
Serpent Wand (if you want to complete the Harespia/Orb of Clarity quest without "whacking" her for it)
Shroud of Elsera (obtained in Shurugeon Castle, needed in Dragonspire)
Dragon Plate Armor (obtained in Stout Mines, needed in Dragonspire)
The Mavin Sword (duhhh! obtained in Dragonspire, needed in Cet's Pyramid)
The Staff of Death (obtained in Skull Castle, needed to open Anephas' Shrine and can be used in Cet's Pyramid)
The 3 Tablets of Cet (obtained in Collasium, needed in Cet's Pyramid).
The Rat Skull (obtained in the Crypt, needed to be buried in the Pile of Sand outside the Anephas Shrine
) (okay, just kidding on that one!)
I think that's all, but I may have missed some of them. Also: after their final use, you can drop those items; for instance, after using the Masque of Death on Haleabus in Castle Shurugeon, you can drop it.
All other items may be dropped at will. Do save the Tomes of Creation(Fire, Wind, Water, Earth), though, if you want to become a Zenmaster!
-Sazerac
-Sazerac