I'm not to familiar with the tint coming as stock but even if they did they usally don't come as dark as you want them to be. So I guess it would be more of any options so you would have some say in what percentage of tints you want.
I will most likely have the rear windows tinted if they don't already come that way but no darker than the tinting that my wife's and most SUVs come with. I had front windows tinted on one of my previous cars and I found it very difficult to see out when it's dark.
I hope the windows come tinted with no extra fee attached, the leather seats will get really hot in summer, so hot it might burn you. Not sure if Mercedes already does it but it would be a good idea if we can choose what percent tint we get while ordering. At least this way it saves us the trouble from having the windows retinted to a darker or lighter tint.
The rear window is a must for, I think it looks best with the rear window tinted to the darkest out of the rest. Darker tints have its pros which is to protect the interior from uv rays especially the leather and keep temperatures cooler. For the CLA I would just get them from the factory, don't want out cheap out on tints.
The only downfall of dark tints is when your driving at night. Visibility is an issue especially when you backing up or looking through the rear windows. A back up cam will be a nice touch to aid the visibility of the rear.
The different shades of tints on all the windows does affect the appearance in my books. i would want all the windows on a sedan to match so it has good flow. But unless you go with a lighter say 35% tint you might get hassled by the police.
i have experienced the issue with visibility at night with tinted windows. the more darker the tint the more harder it is to see out of. And most commonly the rear window is the darkest. Like Benzino said a back up camera would be the best solution.