Not much.
I reserve the right to use plus/minus at my discretion. As an experiment, during the 2004-2005 school year I kept track of how many grades I would have assigned differently if I had plus/minus. I assigned somewhere around 80 grades. Four would have been different. Two would have been higher and two lower.
In general you are much better off worrying about achieving the goals of the Department of Mathematics and Computer Science Grading Standards than worrying about how or if I will tweak grades with plus/minus. I don't curve grades and I don't worry about gpa. I assess your work to the best of my ability, giving you feedback that I hope pushes you to learn and achieve at your highest level.
My syllabus indicates the weight of each portion of the course assessment. Using that weight and the points you earned in each portion of the course assessment, I generate a final percentage for you. After doing this for over ten years, I'm pretty good at assigning points to my assessments such that 90/80/70/60 corresponds pretty well with the levels of achievement in the departmental grading standards. To provide a safety net to the student, I promise that achieving at least 90% will result in some form of an A, at least 80% will result in some form of a B, at least 70% will result in some form of a C, and at least 60% will result in some form of a D. I also insist that you must achieve at least 50% to pass the class. (This leaves a percentage between 50 and 60 murky and I advise you to either avoid that percentage or work really hard despite that percentage being the result.) I also reserve the right to lower the line as I see necessary. For example, there are occasions when, looking at the final percentages, I realize that the students at 78% achieved at a level deserving some form of B.
Plus/minus modifiers on grades will be used solely at my discretion. Your tuition pays for my honest assessment of your overall performance. In cases where I believe the plus/minus modifiers help me give you a more honest and accurate assessment, I will use them.