I never ever turn any of my kit off, PCs are on overnight. This gives them a chance to do things such as overnight software updates, disk maintenance, virus scans or scheduled backups at a time when it won't be an annoyance for the users. Doing software downloads overnight means there is no annoying network slowdown during the day.
(In my case, doing overnight downloads is 400 times cheaper than during daytime office hours in terms of network charges.)
A lot of pcs can go into a kind of low-power snooze mode that saves a huge amount of energy, turning off the display and spinning down any mechanical hard disk, but the machine can wake itself up on a timer or when the network card wakes it up.
Some PCs can go into an extremely deep sleep where the ram is dead and the processor is not running, but can be set to boot at a particular time of day such as in the early morning, which gives them a chance to get various chores done when no users are about. Dell machines can do this. This gives 100% power saving for the part of the night while it is "off". The only bit that is alive is the clock and a timer-controlled wake-up circuit.