Every once in a while, my inventory becomes quite full and I need to store certain "un-needed" items. I'm pretty sure it works but I need to be reassured... I can store items in a closet, chest, etc and they will always remain there, right?

yep sure thing..my favorite place is around the copper coronet [img]smile.gif[/img]
but if you leave it on the ground i think it may vanish but im not too sure about that one

Yes, they will disappear. I recommend leaving it in your stronghold, if you haven't done your stronghold yet then leave it in an inn.

Be very careful about stashing your stuff in certain places as the game swaps the map for a new one once you've completed the quests associated with that place. For example, once you've done Nalia's Keep the old troll-infested map is swapped for a new one, troll-free. What this means is that if you have any items in the old version, they are gone forever.
The best bet is to designate a place like an inn as your "base of operations" and stash your unwanted loot there. A godd place that I know works (cause I've used it myself) is the Copper Coronet.

Another (temporary) bad places:
The Ranger Cabin - the map changes after completing the Ranger quest.
The Bodhi's Lair - the map changes in Chapter 6.
The Thieves Guild - the map changes after accepting Bodhi's invitation.
All these places change only once in the game. Therefore, it is safe to store your items in the Ranger Cabin - after finishing the quest. It is safe to store your items in the (cleansed) Thieves Guild - after accepting Bodhi's invitation. It is safe to store your items in the de'Arnise Keep - after finishing Nalia's quest. And so on.

My base is at Waukeen's Promenade in the old Cernd's house. The house contains several chests at a small place. It does not change during the game no matter whether you have Cernd in your party and do his quest or not.

I stored my stuff before I got my stronghold in the chest in the secret lich room in the Gates district.

A great place to stash stuff is near the Adventurer's Mart in the Promenade. Go up a couple levels, NW of the Amart door and there's a "hole in the wall" behind a barrel of yellow fruit. I find that this is an awesome dumping place since it's so close to a place that a return to regularly.

Is it safe to store items in the Cowled Wizards cabin in the Ulmar Hills?

That's where I've been storing them... but is it safe to store them there?

yup, after u do his quest of course (jsut to be on the safe side)