The best points have been made but I will add a quick note...
The best way that I have found determines whether a press will really help is holding a book in the correct lighting. This exposes it to indentations and such that you may not see otherwise. These indentations, many times, can be pressed out. Same thing with non colour breaking bends and bumps and such. Otherwise, if colour is broken and the book is flat, it is unlikely a press will do much at all to the grade. It was probably a 8.0 before press and after.
However, one area I feel does improve a lot of books is cleaning, especially silver and bronze age issues from the late 60s and through the 70s. Notorious for having white back covers and for collecting any and all dirt, much of which can be cleaned away to visibly improve the book greatly and likely also help towards a grade bump.
Edit: When I say cleaning, I mean a dry cleaning with no chemicals.