There is no right or wrong way as to when to use a bound item. Sometimes I activate them first, before casting spells, especially if I plan on casting some attack spells. If you have two Corpse Carts, the Staff of Damnation, Book of Arkhan and the Rod of Flaming death all being activated at once before spells even go off your opponent will wonder what is going on for sure. The SoD is great when you have Blood Knights, Wraiths or Grave Guard in combat, the RoFD is great against enemy fast cavalry and small annoying things which fly. The Book and CCs are self explainitory. Then throw in a necromancer or two with power stones casting Dance and Curse of Years/Wind of Undeath and you have a very nasty magic phase. Even a demon tzeentch army (which I also play) with 9 dispel dice will not be able to stop them all. Anyway, as I said, there is no right time or answer. It all depends on the situation at the time, and that situation will constantly change from moment to moment.