all depends on what you have.
if you have a laptop, powering down the hard drive is essential for prolonging battery life.
it also makes your computer alot quieter in standby mode
however if its constantly powering down in standby then up again when u come back you are needlessly wearing out its start stop cycles, and its temp keeps fluctuating from cold to operating.
if u arnt concerned with noise or power consumed (e.g. desktop in its own room) then you may as well leave it on all the time. that way it stays warm and ready.
unless of course your going to be gone for ages.
My Hamster has 512MB of SDRAM @ 150Mhz CAS 2!