I don't know about other cars, but mine sat in a sea of cars that had been sitting out in a back lot, in the hot sun, with bird crap and abrasive dirt swirling around. Before that, it sat on a dock for a while. Fortunately, a new car is given over to a minimum wage worker to "detail" your car for you when you buy it. For mine, they used a very abrasive wheel cleaner, which stained the clearcoat around the wheel wells. As I ran my hand over the paint, I could feel the fine grit and it didn't just wipe off with a microfibre cloth. The whole "detailing" process took 15 minutes while I waited in the office.
So, to me, the best option was to start from the beginning with a full detailing process. That way I know that I'm not just waxing over months of dirt, tar, bird poop and so forth. Unfortunately, I don't have a lot of faith that the quick water rinse that the minimum wage worker uses once a week while on the lot...