We never done it on our 2001 E320.
I wouldn't bother with fuel injection cleaning unless you have some serious
drivability problem. Most of the time, when the injector starts to have
problem... like leaking at the seal... is when you should pull all fo them
out... send them out to have it bench cleaned and tested... and then
reinstall with new seals.
It is good to use fuel injection cleaner once in a while... like once a year
or every other year... Lubro Moly Jectron and Venti Sauber is good stuff...
Chevron Techron is MB's favorite... but basically, anyone will do a good job
to 'extra' clean your injector.
None of the gas tank or the engine injected fuel injection cleaner will
match the bench cleaning and testing.