Courses beyond the second year Would it be a poor decision to take additional courses (one each in the 5th and 6th semesters) in physics? Should I try to limit myself to taking only courses that are strictly necessary, or is there value in enrolling if it is related to my research?
If your advisor has no objections to you doing so, and you believe the courses will be beneficial to your development, you should do so. Note that the courses need not be directly relevant to your current work—they may be somewhat outside the scope of your research, but might represent an area you'd like to explore in the future. However, don't take courses you don't want to just to pad your transcript: it really won't matter much to future employers (except a few non-academic employers who might care what your graduate GPA is!).