Age old question - orientation of HDD. will not effect read / write performance and General no difference for under 3 years. The question would be long term bearing wear. Bearing wear would probably be less (more uniform) in he Horizonal position and that is what orientation I generally use. Horizonal is what is used in the vast majority of internal desktops and in all laptops.
You identified one big Plus - less likely to knock over, and YES I've wiped out at least 3 HDD that were vertical and knocked over.
However, I have some very old system at work that ave the HDD installed vertically. These drives are around 15 years old and are stil in use. There are high end SCSI drives and as such were manuf to a higher quality than current consumer model HDDs which I dought would last much passed 5->8 years no matter what orientation.
Bottom line - GO with what works best for you. If vertical just locate to minimize the possibility of knocking them over.
Oh and one more question. If i wrap the hard drive in Styrofoam or something will that lead to overheating and damage it in any way ?
Should i wrap it in Styrofoam to prevent any damages ?