Registering S/D/F

One of the most common reasons students wish to register for a class "S/D/F" is because they are concerned a course outside their primary interest (i.e., major) is too difficult or may detract from other courses they feel are more important.
Registering for a class on an S/D/F basis is usually not the right approach to this concern. Here's why.
Taking challenging courses outside of your strengths or major is normal. In fact, a major advantage of the individually advised curriculum, and of a liberal education generally, is the opportunity to develop and exercise mental powers that are not tied to your major.
Long experience - indeed, longer experience than my own - suggests that registering S/D/F is counterproductive. Students often get a false sense of comfort and drop or put off work. After all, they think they "only" need to get a C. However, it is easy to lose track of things, and the difference between a C and a D or F may not be all that much. This casual disregard commonly leads to academic trouble.
Moreover, many folks reading transcripts will read an S as a C, even if you did outstanding work. Thus for many observers (e.g., employers, graduate schools), an S, does not add strength to a transcript.
In the extreme, if you are not going to put the effort into a class, you may be unlikely to do enough work for a passing grade. In short, either you do enough work for the class, in which case you ought to aim for a real grade, or you don't do enough work for the course, in which case perhaps you shouldn't be there in the first place.