As long as the power is plugged into any Apple laptop, it won't run out of juice obviously. However, don't leave it on power forever. Cycle the battery at least once every two weeks or so (use it solely on the battery until it runs almost, but not completely dead).
I've got an Intel iMAC and I normally leave it on all the time. Whats the best policy?
Is there a reason you can't sleep it? The is little reason to keep it on if it isn't in use and won't be needed in the near future (next hour or so).
I probably wouldn't shut it down all the time, as that tends to be hard on some of the internal components, but there is really no down side to sleep (unless your sleep-mode is flaky and doesn't recover).
The policy that seems to get passed around most often:
Sleep when it won't be needed in the next few hours.
Shut it down when it won't be needed in the next few days.