Continuous heatup-cooldown cycles wear the components, esp. processor, out. If you turn your computer on-and-off around 1000 times a day, yes, it is bad. But whether you turn your computer on a couple of times a day or have it running 24/7 probably won't make a big difference...and it's also good to think about energy consumption. Electricity is not free, you know.
I wouldn't have my computer running 24/7.