In closed loop, that is when the engine is warmed up and operating normally, the O2 sensors will send signals to the computer to fine tune the exact amount of fuel needed to attain the ideal air/fuel ratio of 14.7/1.
The computer has a profile of fuel management stored that came from design and testing at the manufacturer. Of the actual fuel profile differs from the stored profile by a given percentage (around 20% IIRC) then a DTC will be set and the check engine light will come on. A small difference is to be expected and will change as the engine ages.
You will not get fuel wash down unless you start seeing misfire codes. If you don’t have a check engine light, do not be alarmed by a small negative or positive LTFT.