If most students have the same score on an assignment (like on a rubric), is there a quick way to enter those scores?

Yes. There is a quick way to fill in the same score for all students in PowerGrade. This is particularly useful, for example, when you have assessed students using a rubric, and most students have earned a "3". There is a "Fill" feature that can save you time. This is described on page 19 in the purple PowerGrade Manual. Check it out!