To echo the above, put just enough effort into your coursework so you don't look like an idiot. Too much time on coursework means you're not researching enough. Research gets jobs, grades don't. Same advice for TA work. ETA: Forgot that grades often matter for scholarships. At my university you'd want good grades in the first year or two, until you get a good fellowship. Once you're funded they don't matter and most people take them pass/fail from that point on.