Based on what I found in the forums, PR managers have 2 functions: reducing the loss in attendance after losing in the last game before your home game (especially to your rival), and reducing the negative impact on attendance by relegation.
I don't know how much impact it has on the relegation part, but I can tell you that I have a level 1 PR manager, and I've had games where I lost to my rival (away) and had a boost in ticket sales in the following home game. As long as you aren't going to be relegated, don't bother hiring one.