Should I leave it on or turn it off?
Turn it off when not in use! You turn your TV, radio and other devices off so shut that computer down! There are always 2 sides to the issue and I believe the reasons to shut it off outweigh the reasons to leave it on, this includes leaving your system in "sleep" mode. There are exceptions to the rule but these people know how to configure their systems and use backup power sources.
Another reason you should shut your system down is memory leakage. When you run a program it grabs a chunk of memory to use and it is suppose to free up the memory when you close it, but sometimes it doesn't release all the memory and it becomes unavailable for other programs to use. When this happens your resources may become low and your computer sluggish. Re-starting the unit will solve this problem. When you just place your system in "suspend" mode it won't refresh your memory.
TURN IT ON WHEN YOU USE IT AND TURN IT OFF WHEN YOU DON'T