My experience is do not show any documents except invitation letterstating your parent visit wil not add any financial burden to you.

3 different time my family menber got visa to visit us when we were in graduate school(mom, brother, inlaws)

It is their job to ask bank statement etc....not your. If you add more documents that shows you are inconfident and easy for them to reject. Let them review by your letter and ask aditional documents.