It doesnt make any difference to the hard drive which position its in except upside down possibly...

Circuit boards dont really care what kind of an angle they are in... CDROMs can be used on their side, they often have clips to retain the disk... (Playstation, XB360????????????)

The only thing is that cases that are designed to support the wegith on their feet may sufferfrom warping...

Of course somone will now chip in with a counter argument.