I am just getting ready to enter my PhD program. I met with the director to talk about 1st year courses. He made an interesting comment (that I later found out he made to another student when he was entering the program).
The gist of it was this....
Most students have an "all or nothing" approach to coursework. They don't know how to put in any less than 100% effort. They try to be perfect. Some students can "modulate" their effort. To see what is required and just do the right a
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.