Normally, you never need to modify priorities.
There may be no harm (unless you make priority two high blocking important thread, even of the OS), but usually no use.
All attempts I ever so was due to misconception. People devised wrong design first, faced with problems. To solve these problem they attempted to modify priority and got some improvements. In fact, the improvements were illusory: they only changes their race conditions.
Read on race condition, it may help a lot:
http://en.wikipedia.org/wiki/Race_condition[
^]. Years ago I used to read about same thing in much more definitive terminology: "Incorrect dependency on the order of execution". Does this term provide a hint for you?
—SA
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