My perspective on it is that it depends on what stage of learning you're at. A good understanding of the theory can negate any need to do practice exams as you'll be able to cope with any situation. I didn't do any practice exams for BM and Methods, of which were basically my highest scoring subjects because I knew that I understood the theory well enough to not need to do any practice exams. Practice exams mainly iron out careless mistakes (so definitely use them if you're prone to them) and aid in identifying fill in for any gaps in understanding, so if you don't believe or don't know whether you fully understand all the theory, then go nuts on practice exams. Obviously I didn't do physics, but I'm pretty sure this applies to most theory (commerce/maths/science etc) subjects.