The quest reward ones?
I have mine stored on a mule in hopes there will be a place to display them in a house someday.
Lag is ruinin' my 'mershun! A society grows great when old men plant trees whose shade they know they shall never sit in. There is only one good, knowledge, and one evil, ignorance - Socrates Member of the Old Guard, keepers of the game's history
I keep mine now. The ones I got with my older characters I deleted after a while when I started to run low on inventory/bank space. Now that all characters' inventories and the bank are maxed, I tend to keep them.. you never know if we might be able to display them in a house (like Xendyn said, too), someday in the future I imagine it to work like the museum in Orsinium.
I've had to destroy them,sadly.Some day they'll have a tab for them.You do the "use"bit and it records that you got them,so you can delete them from your inventory.Then,I'll start a new character and get them all again.
I always feel bad when someone gives me a quest reward, and always makes the last paragraph some big description of how important the item is to them that they're giving to you, and they're completely sure you'll make better use of it than them...
...And I turn around and deconstruct it or sell it to a vendor for a few gold.
I used to keep them in my bank, but ran out of space, and I hadn't figured out mule accounts back then, bit late now.