That depends on what your definition of a “good boss” is. Most employees define a “good boss” as someone who is patient, understanding, easy to work with, and, most importantly, able to keep their paychecks coming. That may be at odds with your own understanding of what a “good boss” is: that is, someone who's willing to pitch in with tasks well out of his pay grade, to show his employees that doesn't think he's better than them.
Of course, there's nothing wrong with making the occasional gesture to lighten your assistant's workload. If she's having an especially bad day, it won't kill you to spend ten minutes photocopying your own documents. But if you spend more time doing her job than your own, that means you're wasting billable hours, and shortchanging your balance sheet by hundreds of dollars every day.
You should also keep in mind that your employees won't necessarily have the same attitude toward administrative tasks as you do. Your assistant probably considers it part of her job to rearrange the filing system, and the successful completion of this task will make her job a lot easier in the future. Therefore, there's no need for you to feel guilty about assigning her this chore, since a) it's what she was hired for and b) you have better ways to spend your time, in order to pay her salary.
However, there is nothing wrong with improving your leadership skills. We can all get rusty from time to time, and overlook many concerns in our law firm. Keep in mind that being a good leader isn't about making concessions for employees, but rather empowering our employees and allowing them to make decisions within the parameters of their job. Being a good leader also mean setting performance standards and applying those to all our employees, holding everyone accountable for the work they do.