Dal,
As with all things there are varied levels of safe:
(1) Unless the drive is operating outside of the manufacturer's specified temperature/operating/shock range, leaving the drive on 24/7, having it in constant use, does not diminish its longevity (as measured against drives that are turned off/on). <1>
(2) Having said that, there are other consideratiions that come with any other electrical device: component failure leading to fire*, wasted electricity, etc.
(3) In some situations, it might be possible to catch a drive "going bad," and get data off of it before that happens. But only if you're around, know what to look for, and have a specific sort of failure.
Cheers,
Sabot
(highly improbable, while laptop batteries do catch fire on extremely rare occasions, I could not find anything in IT literature to suggest that HDDs have ever caught fire).