I leave mine on all the time, even if away on holiday. I don't want clogged heads and have to clean them manually (cos I'll probably stuff it up! )
The hard drive may not be spinning but I can hear a fan or something going all the time, even when sleeping (the printer that is, not me!). Not sure why it needs the fan, sure yes, when the printer runs maintaneance every few hours, but not whilst sleeping. A bit of a pain, but hey ho.