There's eight drives in my system and they're on 24/7, never sleep them once. When it comes to noise I think the power amp makes more noise with it's active cooling.
"put drives to sleep when possible" when I'm casually working... browsing the internet, light photo work, email, etc; and use DiskSomnia when I'm editing video so my drives don't spin down. 4 internal, 6 external (eSATA, FW800).