You'll see this topic discussed in other threads ad nauseam, but the bottom line is that Daimler chose smaller wheels/tires up front to decrease the chances of the car oversteering (tail coming 'round) and to some degree for better for snow/ice traction.
You can put the same size all the way around and it will definitely improve handling, ride quality, aesthetics, tire selection, etc., but there is a slight risk that in an emergency situation that the car would behave differently than intended. All the electronic nannies (ABS, ESC, TCS, etc.) should keep everything in check, but there is always a small chance that something will go south and it will be on you.
Just remember that all wheel/tire choices made by the manufacturer were a compromise between cost, safety, ride comfort, performance, fuel economy, etc. If you make a change that improves one or more of these characteristics, it may degrade the others.
Having said that, I ran 195/50 tires on 15x6 rims front and rear for 50K miles and have now run 185/55 tires on the same 15x6 rims for nearly 15K more miles and I've never had a problem and didn't see any change in fuel economy. I do, however, enjoy the improvements in all the above. And, remember, the front stock wheels are only 4.5" wide, so you are severely restricted in the size tires you can safely run. It's better to buy two more rear rims to match or just swap out all four for something aftermarket. That's what I decided to do.
