Both of my servers run 24/7, the only time they are off is once every two months for cleaning fans and such -- Or, if there is a power outage, whichever comes first. One is my webserver, and the other is a Folding@Home machine and also the file backup server for my network.
Never had any problems with leaving them on. It puts less stress on the components (heat up = expand, cool down = contract, when turned on and off) when left on.