As far as I remember, I have not come across any such rule. I have seen a number cases in which subordinators are used without comma.
I do not like to write my GMAT now although I am well prepared.
I do not like to write my GMAT now because March is an inauspicious month.
In both the above cases, the sentences are perfectly grammatical. But look at them another way.
Although I am well prepared, I do not like to write my GMAT now.
Because March is an inauspicious month, I do not like to write my GMAT now
These are also grammatically correct. At best, we can say that when a complex sentence containing a subordinate clause is preceded by the main clause, them perhaps a comma becomes essential.
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