Thunderbird defaults to Sent (but you can change it)
Outlook defaults to "Sent Items" (and you can't configure it)
Our company's product defaults to "Sent Items" (and you can configure it, but its what everyone's used to)

So, we settled on "Sent Items"

I've changed the attribute on my account (for a test) and on the COS, and it puts the sent message into the correct folder.

However, I'm left with a "Sent" folder which I can't delete, and which appears "Special" in the Web UI, and isn't deletable. Is there anyway of removing it?