Well copper does tarnish. Look at your pennies. Might come a time when a sufficient amount has built up and you observe inconsistencies in the sound. Also, you run the risk of a stray wire, and there's plenty of threads on this, shorting out the other side. Some units handle this with grace, others may have consequences.
Bare wire is a sloppy approach and you'll not see that as a general rule. Not in your computer, your car battery cables, the wiring harness, electronic equipment, and so forth. How you terminate the wire is pretty much up to you. I happen to think that people who poke around their system are best served with banana plugs on their amps or receivers. For the speakers, you can also use banana plugs or spades. With spades, you've got to make sure the opening is big enough to get around the speaker connector.
There was a recent thread about banana plugs. Do a search or if you need a source or two, I'll dig some up. Locking ones are kind of nice
