Warren,
As far as I know there is not a way to do this in the system (that would also change for all instructors on the system), but here is easy way to do this for your class. Set up an assignment as a template. You will set up an assignment with the grading defaults you want, then hide it. When you want to set up a new assignment, duplicate it, unhide the duplicate and change the name and any other settings you need to change for the new assignment, then move the new assignment to the topic you want put it into. The nice thing about this, is that if you have an introduction or description that is the same, you can save that in your template as well so you don't have to put them in each new one. You can also easily import your assignment template to a new course if you want as well.
B-