Jester,
There is not a thing wrong with your testing and conclusion.
However, you are placing a lot of confidence in the headroom built into the servo by the manufacturer.
And, you did not test that headroom.
Additionally, since it is headroom, it is apt to vary from manufacturer to manufacturer and from batch to batch.
Many people have had trouble with servos on five cell Nixx packs as well as 2 cell LiFes, many people have not.
It is a personal preference if you regulate or not, there are pros and cons to both.
KW_Counter