Normally, I encourage my students NOT to rehearse in front of a mirror. (My approach to acting is process-focused, not results-focused.)
Now I am of the opinion that it depends on what you are rehearsing for.
For example, if you are rehearsing for your headshots session, then I think it's extremely important. (Because of the results-based nature?)
My approach/belief is that acting results are better when you focus on process and allowing that to produce “surprise” results.