A few things:
Here in Arizona, tint is more of a requirement than in other areas. It helps the A/C not work as hard to maintain a cool temperature.
I didn't add shaded tint to my Land Cruiser front windows for a couple reasons. One is because the factory privacy glass is a slightly mirrored bronze. Matching that on the front seat windows would be next to impossible besides, mirrored tint is not legal on the front seat windows in Arizona. The other is because my 80 is also my hunting rig, so I am looking out the window as we travel to different areas, and I am constantly scanning the forest. It has a UV film that is clear to cut down on the rays. I have 5% shade over the factory privacy glass in the back to keep prying eyes off my gear and to keep it cool back there. It cuts down on my fridge cycling and the A/C circulates to the back passengers. (No rear air on US 80 series)
On all my other cars, I have Arizona legal limit. 35% front, and as long as you have a right hand outside rear view mirror, you can paint the windows behind the driver's shoulders if you want. I only go to 5% in the back on anything because I still want to see out.
The tinting in factory privacy glass is only that, tinting added into the glass as it is manufactured. There is a very slight UV reduction, so if you have little ones that are in the back, you should at least add the clear film.
I have found that you can go a little too dark. We have always had a mini van or large SUVs (Yukon, Suburbans, Cruisers) and I had 5% over the privacy glass on all of them. However, with two kids going off to college, we decided to downsize the wife's car, so my wife bought a new Honda CR-V. I took it to my glass guy to have it tinted like usual, and let me say, the 5% over the factory glass on a small car makes it very dark in the back. It takes a little getting used to. Fortunately, it is a small car, so backing up isn't like backing her Suburban. I may have the liftback glass redone in 15%.