Sailorwoman the answer to your question is NO. As with everything else this is subject to each manufacturer. Just like with rulers. An inch on one ruler may be off a bit from a different brand of ruler. Just like in clothing, one designer's size 16 will not be the same as another designer.
Now, is it alright for you to change this setting? Yes! Remember, YOU purchased the machine. The manufacturer is not paying you to have that machine. Plus there is not one word in your warranty that says you cannot change any settings on that machine. If there is, you need a different machine.
I even change my bobbin tension (OMG THE BOBBIN?) if I wish to run a heavier or thinner thread in the bobbin. Once again, DOES NOT AFFECT THE WARRANTY! Also, that machine belongs to you, you paid for it.
If EVER you have a tech that says changing these things will affect the warranty, challenge that person. Ask them to SHOW YOU where that is stated! Once again, if it is, get a different machine. (Not that I've had this happen to me hehe)
As to where do I set my upper tension, what ever works best at that moment. On days that are dry and little humidity, I have to set the tension a bit higher. On humid days, just the opposite. There really is not a one size fits all tension setting. Too many variables in the equation.