theres arguements either way.
in the end who really cares, as long as they work for at least 5 years im happy.
leaving it on 24/7 means there'll be higher chances of it be subjected to power supply events like over voltages and spikes, which is a problem where i live :\.
also being on 24/7 stresses the mechanical parts more - such as harddrives or fans.
fans especially have a tendancy to wear out (especially shit sleeveless ones :\),
im unconvinced of thermal expansion and contraction being a factor that is worthy of me caring about. though theres otherthings that i deem unworthy of my attention that disagrees with other peoples practices - such as having a av program
but hey...i encourage people to leave their computers on, the only real downsides i think you should consider are the increased powerbill and the respective impact on the environment youll have by using more electricity.
it sure is nice to start writing something, then not bother to close vim and come back the next morning and have it open and waiting for you