As with opening any account or tradeline, your credit score may initially show a slight decrease. Everyone's credit report is different and subjective to many factors. There are so many variables that affect everyone's individual credit score, this value is not one we can predict.
Rent reporting builds history in your credit file without adding debt. Because rent payments demonstrate responsibility, and the ability to make large on-time monthly payments, the scoring models take that into account.
Remember, credit is all about demonstrating history. So, typically, the longer you have a rental tradeline on your credit report, the better.