It is possible to obtain quite a good score if you know most of the topics already. You should absolutely do the 5 available practice exams and check what topics you do wrong to see where to focus your study. For example, I noticed I was quite weak in using spectroscopic notation, selection rules (both of which I didn't quite know by heart when I was doing the actual exam), EM fields in matter, resonance in RLC circuits and dipole radiation (which I did quite well on during the actual exam). Of course, the topics you need some extra focus on depends on your background.
Now, back to your original question. The PGRE is usually required for PhD. programs in the US. So if you are using it as a diagnostic tool for your knowledge & level of physics, it might be better to just do the 5 practice exams, and not take the actual PGRE (unless you have some extra $150 dollars to use).