It's up to the officer's discretion. I used to have the smoked plate covers on my old SE, and two times, with my wife (soon to be former) was driving my SE, she was pulled over. Officer said that she (and he on other occasion) could not see the plates from across the intersection. Both occasions were in Peel Region, once by Square One, other occasion by Peel Regional Police Headquarters.
I then removed the smoked covers and replaced with clear ones.
BTW, she was pulled over due to having a non factory exhaust system and Xenon headlight bulbs (both times), lucky for her, the ownership was in my name.
