It is true that the amount of light hitting the interior of the car will affect how visible the interior is through the tint.
It will not mask the fact that the fronts and rears are drastically different shades.
I had 30 up front and 15 in the back on my RSX. The different was noticeable no matter what the lighting. I wanted a more uniform look so with the WRX I just got the same level (30) all around.
I also didn't want the back to be as dark as they were with the 15, as I had problems at night-time when I was backing up at times.