Nissan, as a company, has always tried to do things a little more "right" than other manufacturers. Toyota might focus on making cars marketable and reliable, Honda's got the market cornered on non-hybrid efficiency, and Mazda's held onto "crazy" like it held stock in the concept. But Nissan, like BMW, builds great engines and great cars, like the Altima, just to build great cars.
The difference between a 215/60 and a 205/60 tire is that the latter uses 10 mm -- about 1/2-inch -- narrower tire tread, and sidewalls that are about 4 mm -- 1/8-inch -- shorter. Generally speaking, installing a slightly wider tire on a given rim is playing it safer, since it reduces the chance of the tire coming off the bead under hard cornering or low air pressure. However, it's not uncommon to mount tires on rims that have exactly the same tread width as rim width; manufacturers will generally mount wider-than-the-rim tires for more absolute cornering grip and ride comfort. In this case, the stock 215/60 tires have tread measuring a little over 8 inches wide, which leaves plenty of room on a 6.5 to 7-inch rim to go down by 1/2-inch to 205-series tires. Expect a slightly harder ride, and a bit less grip during sustained cornering maneuvers. In exchange, those shorter and stiffer sidewalls will give you sharper turn-in and steering response, and more feedback through the steering wheel. The car's ultimate handling limits may be just a tad bit lower on a racetrack, but it will feel sharper and more agile around town.
