Years ago when the conditional Permanent residency rule came into effect, my wife was subjected to it. At that time, not many people had even gone through the process. I had gone through the application and decided to provide 1 or 2 proofs in each category listed by INS.
I do believe that there is such a thing called overkill in submission of proof.
The application contained the following:
1. Copy of birth certificate of our 1st child
2. Copy of the mortgage that was a joint responsibility.
3. Letter from the bank regarding our joint bank account
4. Copies of joint tax returns since my wife's arrival in USA
5. Two identical affidavits from my parents ( we lived with them )attesting to existence of a bonafide marital relationship between the two of us. The affidavit further attested their willingness to testify to the information in front of an INS officer, if asked to do so.
6. Ofcourse the I-751 duly filled out
We got out approval in the mail in a few weeks and then we got the stamp at the local INS District Office.
If you see the instructions for this forms, the above info covers all categories requested by the INS