From the "Rocky Mountain Home Energy Brief"...
Arguments that thermal cycling harms the internal
components of computers don’t seem to be well
founded. Modern computers are designed to handle
40,000 on-off cycles before failure, and you’re not
likely to approach that number during the average
computer’s five- to seven-year life span. In fact, IBM
and Hewlett Packard encourage their own employees
to turn off idle computers, and some studies indicate
it would require on-off cycling every five minutes to
harm a hard drive.
Now, this quote is taken from a study done in '95, so how the numbers may have swayed in one direction or another. But I do imagine 'sleep' modes to be sufficient for daily uses. A hardware designer once told me that sleep is good for the work week, but if your computer is out of use for more than 24 hours, it's actually bad for the components to keep them in that constant state of readiness. In other words, more than 24 hours, shut it off.
On another note, any electricity use, as it stands now, necessitates the burning of coal. This is a bad thing. So until we move to renewables like solar, wind and hydro, electricity use should be a concern no matter the use. Remember, its not just our use, but our combined use that's the problem.