N00b, you need to seach first and do some reading here before asking a question. There's a forum called FAQ Emporium that has a lot of good information and will likely provide answers to the majority of your questions.
For some reason, there's nothing addressing this topic in the FAQ Emporium. I'll remedy that as soon as I turn off my flamethrower.
There's many threads on this very subject on these forums, so there's no need to add another one so that when someone DOES search on this topic they have one more thread with little or no information to weed through.
This is why n00bs get all ****ed off. There's so many damn threads to read through that it's difficult to locate worthwhile information. You, sir n00b, have increased the problem with this thread.
The answer is this: There is no evidence that mounting the HDD in one orientation or another has any impact on the reliability of the drive. If it's a concern, some additional shick and/or vibration protection in the form of rubber/silicone/nylon washers will help protect the drive.
Friggin' n00b... Learn to search first, then ask questions if you don't find the answer.