in the USA (and australia) its quite common to tint cars - i think a lot of people just do it because it looks cool. If you want a proper european look on your bmw or merc you need to go untinted though - just doesnt look proper european when its tinted. To be honest, in europe its so dark and miserable 8 months in the year that tinting just robs you of extra visibility...
I do think tinting is a really good idea at least in southern USA and in Australia. It probably reduces a bit of eye fatigue driving during daytime and also most tints have UV protection. The sideglass of the car doesnt really block much UVA/UVB (I think for Teslas its... 66%?) so if you drive somewhere where its sunny all the time or in countries with a thin ozone layer i.e. Australia you expose yourself to significant UVA radiation - this does not cause sunburn and is also present in mornings and evenings when the sun usually shines through the sideglass. And dont forget that you might have children in the back.
So I would always fork our a bit of cash to tint the side windows. If you like the european look then you can get a crystalline tint, either 100% VLT or something like 80% throughout. Just dont go too dark or pedestrians/cyclists cant look inside. 35% probably max.