well it depends on your computer wethere its new or not and if its clean.
generally computers are designed to handle being on 24/7 but if their old they might not make it and if they are dirty well i hope you have home insurence.
but if its clean and new leave it on. its more of a convience and just like a car engine startup on a computer is a ***** on the hardware
what happen if you leave it on all the time? will the computr go slow or no?