Honestly, I think that if it hampers your academics I wouldn't do it, unless you have
no other leadership experience. From top tier to bottom, I know many people in medical school who haven't worked an honest day in their lives. But if you don't have any leadership experience and you could still do well while working the 20hr/wk, it would be worth doing to put in your application and talk about at interviews as a unique experience you have.
Maybe it's not true as it's an anecdote, but it seemed interviewers and people in general were more impressed by my academic and non-time-intensive things (e.g. grades, school organizations, honor's, research, shadowing) on my application rather than things I spent 60+hrs/wk (work, extracurricular activities) on in addition to school (40hrs/wk, full-time, 3 labs/semester). I think this was particularly true with work, as my extracurricular activities were brought up a decent amount, but no one seemed to care about how much I worked even though I did so extensively during my education.
It's a bit backwards to me, as I personally have much more respect for someone working a significant job outside of academics and still doing well. I think this might actually be a product of how poorly AMCAS organizes things. It's difficult to see a student's rigor in their coursework and parallel that with their activities at that time. I believe AMCAS should incorporate an application timeline that compiles Courses and Activities on it. This is considering they already do hrs/wk in the activities section. But I doubt they will. With that in mind, I think it's best to limit your work unless it provides something truly unique, you need the money, or it's an on-campus job that limits commuting, allows for studying, is minimal hours, etc.
Would it look nice? Sure. Would it look better to have good grades, because you have time to study and sleep? You bet. By a lot.
Basically, this.
Edit: elaboration