有句话不知该讲不该讲。 In 古代 dramas they used it quite a bit. But I have also heard the 当讲不当讲 version. It has always been in a situation where a lower ranking personnel was addressing a higher.
Are there any subtle differences where it is more applicable?
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A very good question.
该讲不该讲 means "should or should not actually talk about a certain matter"
当讲不当讲 means "whether it is proper / appropriate to actually utter certain words"
The differences are:-
该 = "should";
当 = as an adjective, "proper / appropriate"
In 该讲, you have information that may or may not be true, or be harmful to or cause anger to someone, including the listener.
For example, "Your Majesty, I am not sure whether I should or should not say this, but there is an unfounded rumor that........"
In 当讲, the words themselves are not "proper / appropriate" to be uttered; as it may contain disparaging, unsavory words, "bad words" which are not to be uttered in polite company.
For example, "Your Majesty, I feel it may not be proper, appropriate for me to say this, but the town folks are saying your Majesty is a...(the improper words)"
So, the "ranking" of the speaker or listener to each other is not strictly material here, though of course the lower ranking of the two should exercise more caution or circumspection.