|Computers go bad from time used. If you do have them actually on they could fail more quickly.|
From an environmental, fire hazard, security and hardware point of view you should not.
I put all my computer's and every ac adapter, phone jack, nic jack on a power strip that protects everything. When not in use power it all off..
I know that no power is being used. No fire can exist from any failed part, my ac isn't being used, my carbon footprint is reduced, hackers have a harder time trying to get may data. Power drops or spikes can't damage parts or data.
Look at the reasons you want it on. Sleep and hybrid sleep and hibernate are great choices still. I keep a small atom based htpc on only to record shows at night. It wakes from about 5W use to 22 watts. A common pc could use 100W or more in sleep mode.
Why did it take me over a year to phone in a problem to ATT?