I usually do what JB does. Switched on when needed and left on for the day. That's days in the case of the Internet Gateway that serves the rest of the Network.
I've never had a PSU failure although anything is possible in this business. Monitors are the components that fail the most for me and to some extent I wonder whether Energy Star is doing us a favour by constantly turning off the EHT, only to suffer the surges whenever I wiggle the mouse.
Current surges during startup are the killer of electronic components as all those capacitors and inductive components settle down. By leaving your PSU running you are avoiding them. But by leaving the box running you are using watts, and keeping various components warm and whirring. I guess you takes your pick...