I put those items on my last backpack page, and then any that are still there after I finish a good amount of questing, get trashed. I've only heard of very few quests that are "broken" to the point where you can't actually get an item back if you trash it, and I've personally never had that experience. I think most people tend to be over cautious because of all the hype/stories of not being able to finish a long quest line because of 1 deleted item... to be fair, I'd say what 99.9% of quests don't have that issue? Plus, worst case you can contact support and get a new item after a short (few weeks at most) wait. Except I do try to remember the ones that were really hard to get, like from a boss drop, and try not to trash those.
I mostly quest in bursts though. I quest hard for a few days, then no questing for months... so I'd really hate to keep that stuff in my pack, even if I did need it later. Would rather spend a bit of extra time later to get it back.
some i like to keep just because of some cool thing they do...like that book from ystra labyrinth that makes the gloy thing over you...or some of the zhuridon disguises from AV
One of those items used to be the only way to get even with those dang warden crashers in siege also. I'm sure they've fixed that though, only reason I feel safe to mention it now.