An upgraded forge is definitely not a waste of iron, as it is absolutely necessary for repairing high-end weapons and armour. Fully upgraded Padded Armour requires on average 18 iron for each item. It's not a waste to upgrade iron some pieces of iron armour as it is nice to have a good secondary set of gear for corpse retrieval. I still use my fully upgraded iron leggings and Wolf Armour Breastplate when going on a corpse run. I also always carry 2 fully upgraded iron pickaxes when I am going to strip mine Sunken Crypts.

Because I do a lot of castle and base building I like to have at least a half a chest of iron on hand.