Here are some guidelines to keep in mind when you ask me for a letter of recommendation.
If you have received a grade of A in an upper level class from me, I will happily write you a letter of recommendation for graduate school. But before you ask me, there are a few things you should do.
If we have never spoken outside of class, please stop by my office sometime and tell me about your plans for graduate school, because I genuinely want to know.
Consider that I am not a full professor at the math department, only an assistant professor, and an assistant adjunct professor at that. If you're applying to a top 25 graduate school, I strongly recommend that you get a recommendation from a full professor. I will be happy to write you a letter anyway, but it will very likely carry less weight than a similar letter from a full professor. If you change your mind about getting a letter from me because you got someone stronger to write for you, I will not be offended in the least.
Please collect all your application materials, a CV and perhaps a personal statement and give them to me at once. Please give them to me at least a couple weeks before they are due.
I strongly recommend that you waive your right to read the recommendations. You won't get to see them anyway unless you get in, and at that point, you probably won't care too much. If you do not waive your right to read them, the recommendations will have much less credibility.
I'm not exactly sure what my standards should be for non-math jobs. Probably a B will be good enough for many jobs, but if you're applying for a job at a top firm, you probably should have gotten an A-.
Again, please tell me a little bit about this job, how it fits into your future career plans and how your undergraduate education and experience has prepared you for it. Please give me a couple weeks' advance notice if you want me to write a letter. If you just want to list me as a reference on a job application, just give me a heads-up so that I know to expect a call or email from the firm.
If I decide that it is not appropriate for me to write a letter of recommendation for you, or my letter doesn't get you into the graduate school/internship/job of your dreams, your life will not be ruined. Honest.