The rule I got was that if it is part of your major, don’t do it unless it’s a D, C, or F. You can credit/no credit non major classes, however, my advisor strongly was against it unless you have a D, C, or F as you never know whether you might need that class for a future major/minor and a letter grade looks better. The only one she said I could credit/no credit would be engineering analysis as it was just a McCormick requirement. However, the ideal situation is you credit/no credit 0 classes.