Shorty Final,
I always leave my PCs on, I have PCs here at the office that have been running 24/7/365 since 1998.
Each time you turn the computer on, you heat it up and the parts expand. When you turn it off, the parts contract and there is a chance to get condensation. When the PC is always on, it helps keep out any moisture since it is warmer than the surrounding air outside the case.
Take Care,
Richard