I am using as a reference "MERCEDES W-124, OWNERS WORKSHOP MANUAL, 1985-1995" first published in 1994 by Dellus, Klasing & Co. Bielefeld Germany.
page 58, "IGNITION TIMING"- testing states:
" The ignition timimg does not normally change. It should be checked if the fuel consumtion is too high or, in vehicles with the TSZ system, if the distributor has been removed and refitted. The ignition timing does not need to be checked in vehicles with direct ignition.
page 60 chart " Ingnition Timing Values", indicates that a 300E. with emissions control (catalyst) has the EZL ignition system and is timed at 650+/- 50 RPM with BTDC at 6*+/- 11* with vacuum hoses connected. a timimg value is listed for 300E. @ 3200 RPM@ 25*+/- 2*BTDC, non catylyst models.
The EZL system, in vehicles since 9/86 ( Retrofitted and OEM catalyst) has a EZL equalizer plug with fuel settings that can correct timing for gasoline quality.
In closing, it seems in general, not nessasary to mess with the timing.