Optimal degree of parallelism

Hi,
Situation
IBM AIX 4.3.3
CPU count 4
Oracle 8.1.7.4 Enterprise Edition.
We are setting parallel query in statement level as

select /*+ parallel(emp,4) */ from emp;

Tables are large enough to utilize parallel query.

One of my fellow DBA is telling me:
<QUOTE>

As the general rule, the optimal degree of parallelism can be safely set to n-1 where n is the number of processors. That means 3 not 4.
</QUOTE>

I serached google, oracle documents and metalink for this "general rule" but never found any reference to it.
Usually I set number of parallel slaves in powers of 2 and never set this to a odd value.
Is it a myth or I am missing something ?

Thanks,
Saikat Chakraborty

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