I had found $3,000 to be the consensus when I polled a number of colleagues, but it will depend on your circumstances.
In most cases the $3,000 is sufficient to 'catch' most aggregations that go over $10,000, but not cause too much unnecessary work for the branch staff. If we do get a number of cash transactions for one customer under the $3,000 limit that go over the $10,000, I just give a call to the branch and ask them 'who did it'.
Our MI recordkeeping only kicks in when someone purchases a cashiers check and captures from $3,000 to $10,000 (obviously anything over than the $10,000 will kick-up to a CTR.
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I hear and I forget. I see and I remember. I do and I understand.--Confucius