Everyone has different views on this, putting power savings aside. Who leaves their HTPC's powered on all the time?
I am thinking of leaving mine on all the time due to mainly using it for Freeview HD with Mediaportal.
I have run into countless issues by having it in Standby when not in use, the most extreme is Mediaportal crashing when a scheduled recording is about to start. HTPC not waking up when a Scheduled recording is about to start etc.. Yes I am using the power plugins etc but still not reliable.