Potential Impact of Pass/Fail vs. Letter Grade in Elective Courses
Unless you're applying for a post-doctoral position at a corporation or somewhere else where HR personnel will be screening your materials before forwarding them to the person doing the hiring (where GPA might be scrutinized), I don't think it frankly makes much difference. Your work as a researcher matters much more than your performance in an elective course taken in graduate school that may have little to do with your proposed research area for the postdoc.