I was told the only thing to avoid is moving the hard drive while it is turned on. My instructor told me he's ran into this problem a couple times, where he'd be working on a computer and while it's turned on he'd set it on its side. Then you hear a WAP WAP WAP WAP WAP WAP WAP and then the computer seizes up and dies. Turns out the platters inside were so hot that when the hard drive was turned when the computer was set on its side, the platters created an oblong shape and the wider edge was slapping against the inside of the actual hard drive enclosure.
In short: hard drives should be fine in any position, as long as you don't have any magnets around it and you don't move it around, or obviously knock it. In fact I distinctly remember my other instructor saying the hard drive chamber is sealed airtight/watertight/blah blah so really the drive doesn't even "feel" itself being in a different position.
This is just what I've been told from my instructors, which have a good decade under their belt of experience. Me personally, I don't have any first hand experience from this. But these are the guidelines I'm keeping in mind when I get out in the field.
There's no place like /home.