But wouldn't the most optimised game use all of the RAM? If it's not using it all... then there is still untapped performance?
Depends on your needs. Going out of your way to use as much as possible or as little as possible seems silly either way. Use what you need to deliver the experience you want to, within the confines of what you can technically perform and afford to do.
That's my guess, anyway; I'm speaking from an outsider perspective, but rationally that makes the most sense.