Leaving your Mac on for anytime more than 50 hours could seriously damage the BS infrastructure - contact your nearest retailer immediately, you'll need to buy some Mac coolant, and the retailer should be able to tell you exactly what you need to do. But DON'T turn your Mac off now until you've added the coolant, or the damage may be irreversible...
If you're not going to be using it, turn it off and save some electricity, if you're stepping away for a little bit you can sleep it.
Seriously that old "keep 'em running all the time" thing is probably the real reason behind California's power crisis.
Diesel Trucks and busses. Ever drive behind a natural gas powered bus in San Diego? No smoke.
Server farms, and their diesel generators during rolling blackouts.
Poor design of homes and businesses.
Overtaxed infrastructure, see overpopulation.
California's a mess these days, I moved up to the Northwest in January and I laugh when I get my power bills. Haven't paid more than $12 a month yet.
To have a computer on for a few days is not harmfull, may be in the 80's but remember that when we have one computer at home there are companies and schools, production houses, etc. that have their equipment runing for the entire week even 24 hours a day with PCI cards and memory sims in every slot.
What is bad for a computer and any electronic device is to be off for weeks or months, humedity can harm the unit.
When you come from vacation, tur on your computer just for an hour or 2 before use it.
I work with lots of audio equipment, usually when I turn them on after a few months they come with wird sound and noises. I have a Roland sintetizer module that I didn't use in 8 months for more than 2 hours. I turned it on 3 days ago and it was making wird sounds in the back ground, I thought the unit was **** up. After 10 hours it was working as brand new, I'm sure the internal heat remove the humedity from the circuits, 3 more month and may be I would lost it.
So, heat can resolve most of the problems.
Another tip is not having a fan pointing to the unit at all times, you may be introducing humedity and dust, many amateur audio engineers do that, they push air in place of pull the heat out. If you push air in to an aplifier in a club, after one week you are gonna end up with a sample of every single cigarret, dust, and kitchen oil on the unit.
if it's not getting too hot, then leave it on!
i have an older iMac with a fan and while
it doesn't seem to function less efficiently,
the back/top of it gets VERY hot. my ibook
obviously doesn't have a fan, and as most
if not all laptops it also gets very hot, with
no problems observed. I did have an older
computer though, and by older i mean
summer of 1983, and in the summer it's
display would go from it's standard blue
to a very ugly "help-i'm-frying " shade of
brown. that being said, i think you should
leave it on. get a gnutella client, put tons
of mp3's in your shared folder, get a broadband
connection if you can, and leave your computer
on 24/7! as martha would say if she had a clue
about such things, "it's a good thing!"