The only issue I have with putting the hard drive to standby is that sometimes just as it goes to sleep and the motor spins down, it is needed again and the drive spins up before it has compltely stopped. This is not good for your drive!
I have no figures about how this will affect the drive's life, but I am pretty sure if you do it repeatly (like in a lab environment) it will break eventually.
gster123 is right, just turn off the box when not in use. Hard drives don't use much power anyway, most use maybe 3 to 10 W (no figures, I deleted my collection of hard drive datasheets).
Really? Don't the render farms just go on and on and on with no break for months at a time? I know that there was a computer (a server) that was going for 14 years non stop before the owners replaced it with a newer one...
C. P. U. Its not a big processor... Its a series of pipes!