Technically speaking the 30-60-90 day is not a rule. It more like found in the DOS 9FAM (Foreign Affairs Manual) given to the IOs to assist with adjudicating cases, and it states:
"If an alien violates his or her nonimmigrant status . . . within 30 days of entry, you may presume that the applicant misrepresented his or her intention in seeking a visa or entry. If an alien initiates such violation of status more than 30 days but less than 60 days after entry into the United States, no presumption of misrepresentation arises. However, if the facts in the case give you reasonable belief that the alien misrepresented his or her intent, then you must give the alien the opportunity to present countervailing evidence."
It is understood that a person can enter the US on a NIV and subsequently have a change of heart due to changes in circumstances. If that person were to demonstrate an immigrant intent within the first 30 days of admission, then that person most likely had an immigrant intent prior to coming in on a NIV - IOs are encouraged to deny the filed petition in such cases. If the person where to wait at for at least 60 days before demonstrating an immigrant intent, even if the IO had some doubts about the persons original intent, it may be overlooked, and if the person were to wait 90 days, having a preconceived immigrant intent wouldn't be considered.
It's basically saying the longer one waits to declare an immigrant intent upon entering the US on a NIV, the easier it is to convince the IO one did enter the US with a preconceived immigrant intent. So you make the call on when to do what based on the information you've been provided.