Agreed, I've generally found it better to hand in XP quest items in after you have outlevelled the mobs.
Obvious exceptions being if the items are lore, or you are running out of bank space.
... that said, if it is the bone chip quest in Kaladim you could accrue a huge number of bone chips from Befallen / Najena / Kurns and end up having to hand in a thousand of the things in your early 20s (which is basically trading RSI for an extra level, or more if you were a truly relentless collector)... Perhaps it would be worth selling some of the xp quest items to players for plat instead.
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