I actually have had the same problem. One of the injector was leaking petrol onto the engine block causing blakening marks etc. I was kind of worried for fire troubles more than anything as it did not cause any harm to the drivability.

My mechanics told me to leave it. (it was an early W140 and replacing 1 injector requires unpluging all of them which could cause damage to the plastic of a good injector thus requiring more replacement)

The folks here told me to leave it and buy a fire extingisher.

I went ahead with the replacement and lucky for me, my mechanic did not break any and I only had to replace that 1 injector. Cost 200 USD each!!!

NOW, I don't have a leaking injector that leaks petrol onto the engine block. BUT I have hard to start problem at warm temperature. I search, replace, search and more search, I still have to start the car twice in hot/warm engine!

My advice, its your call. Its best to leave it unless you really have to fix it!