If your Mini were running in a perfect environment -- with clean air and no power interruptions, it might be happier running 24/7. As has been pointed out, OSX likes to run 24/7 and most of the components in the Mini prefer a "steady state" of temperature.
But in reality, your Mini is likely running in dirty air. The more hours it runs, the more dirt is sucked into the DVD drive and onto the heatsink and the fan.
When I updated components on a three-year old Mini, the hardest thing to clean up was the cooling fan blades - because of the dust and grease that had been deposited on them from operating two rooms away from the kitchen.
Hard drives get very unhappy with bad electrical power - particularly brownouts or sudden outages. If your Mini is running fewer hours, it will have less exposure to these.
In any event, I urge people to use a good UPS (uninterruptible power supply) and not just a surge protector on their Mini.
My personal approach is to leave my Mini running when I'm home and awake. I turn it off when I sleep or leave the house. And it is connected to a UPS.
Your question (unfortunately) takes on the nature of a "religious" argument in computer circles. Good arguments can be made for both the "yes" and "no" responses.
