If you want to view the balance of a cold storage wallet, you need to create a view-only wallet. The answer from @ferretinjapan is correct, in that respect, though there is more to it.
For more clarity:
If you want to track the balance of a cold storage wallet to which you make occasional deposits, then you need to create a view-only wallet (the same one as already mentioned, of course).
If you want to confirm that payments haven't been made out of a cold storage wallet, then you need to restore the whole wallet. Your private view key lets your wallet reflect all deposits to the account. Key images allow your wallet to reflect which of those deposits have been spent. Key images can only be created using your private spend key. Since you need your private spend key to be part of the wallet, you'd need to restore the wallet from seed or from keys in order to view the full balance.
You create a wallet and deposit 500 XMR in it. You spend 100 XMR over a few transactions, and then you decide you want the rest (400 XMR) to be in cold storage. First, you'll export_key_images and back that file up on a USB. But wait... you decide to send 50 XMR to an exchange real quick (bringing you down to 350 XMR); but you've already exported the key images, and you forget to do it again. You then format the hard drive, so that wallet no longer exists in electronic form. Cold storage, check. And now you want to create your view only wallet.
When you create your wallet from view key, and then you import your key images, you should see a balance of 400 XMR, because that wallet doesn't know that the 50 XMR was sent to the exchange. If you send that 50 XMR from the exchange back to the wallet address, the view only wallet will think you have 450 XMR, still not knowing that the 50 XMR was spent in the first place.
In order to view an accurate balance in a view-only wallet, you'd have to fully restore the wallet, and then export_key_images again, and then create the view only wallet again.
(Actually, I need confirmation about whether that 50 XMR key image is gone forever, or whether it is recreated by the private spend key upon fully restoring the wallet.)