As a thumb rule it is safe. Finding odds with electronics is always tricky.
Even if the odds are 1 in a million, as far as you are concerned, it is 50/50, may happen, may not happen. Thats as good as it gets. Safe shelf life of a always-on HD is 2 years.
Importance of data backup has already been covered by others.
I'd be interested to know the need for such a thing to understand the cost/benefits. Plugging in is different from 'always on'. Will your laptop be on standby often?
Anything with moving parts is bound to fail some day and in general more likely than solid state devices. So the clock is already ticking once you power on any electronic equipment.
Having said that, tech futuregazers predict that, as $/byte trends down and internet speed increases, the limiting resource will just be time.
2-3 years from now, is the data you have currently going to be irreplaceable ( business data, personal stuff)? or look back 2 years and see how you critical your data is to you.
Another cheaper option would be burning DVDs and putting them in a vacuum sealed bag. Many DYI sites have the how-to.