I'm currently sporting a full set of iron armour as well as Light's Bane and a Demon Bow and an assortment of silver and gold tools. I'm feeling like copper is now totally useless to me, except as a money maker. However, iron seems like it would be perpetually useful as it can be used to make such things as buckets and toilets.
So my question is, do some ores become useless as you advance, and if so then which ones do?
For weapons and armor, they all become useless eventually. Once you've got full adamantite armor/weapons there's no higher tier to mine and there's no point to making more of what you already have. However, even the lowly copper remains useful for watches, depth meters, timers, and hardmode boss summoning items. I'd suggest keeping one hundred bars of each metal from copper to gold on hand and sell the rest. If you want to know what you can make from each kind of bar you can use the guide or the wiki.
Regarding to usefulness, id say yes; copper becomes useless. However, if you plan on decorating your house later on, you can use copper to do "pretty" stuff such as chandeliers. But as material for equipment, yea, it gets obsolete pretty fast
All the metals become obsolete in regards to end game weapons and armor. I however horde the ore and do not make bars. With the ore you can turn it into bars to make items like the depth meter and candles/chandeliers when you need them. BUT ore can also be used to make bricks and walls to create those amazing structures for you and your NPC's to live in.
I generally make living areas out of copper bricks and storage areas out of gold bricks. Bedrooms are generally done with silver brick and I use silver walls for most if not all of the dwelling. Save what you get and make a big storage area to hold it all.
Starting a new level? Well you were just shy of 3 silver bars for your new toons armor. Well with that nice storage area you have now you can go into that world and grab those few pieces of ore you needed instead of running all over the tunnels for it. GL to ya.
As mentioned in the other answers, ores can be crafted into bars which then can be forged into various accessories and furnitures that require them, in addition to the usual set of armor and weapons. In addition to that, ores can be combined with a stone block to make bricks of various colors, which can be used for construction purposes.
Also, iron ore is also required in the making of Ironskin Potions, which raises defense by 8 for 5 minutes, while gold ore is required for Spelunker Potion, which highlights and illuminates ore, gem deposits, chests, pots and crystal hearts for 5 minutes. Both of these potions are very useful even near the end game, so you'd want to stockpile a larger amount of each of these.
