apart from the very small risk of fire etc:
computers are designed to run 24/7 and most people will tell you that switching on the computer from cold puts the hardware under stress as as it heats up and cools down every time you do this(restarts not counted)
my computer is switched on and off once a day. i just switch it on when needed and shut down and switch off and unplug last thing at night...usually.sometimes its left running.
i set my monitor to go into standby after half an hour of inactivity and the hard drive to spin down after two hours of not being used.
use a surge protector(cheap) to keep your equipment safe from damage.
fit a smoke alarm.