The Boolean setting of $FCShowIEta determines whether i \eta should be displayed in the typesetting of propagator objects (except for FADs) or not. This setting affects only the TraditionalForm typesetting and has absolutely no influence on the internal handling of propagator denominators in FeynCalc.
$FCShowIEta
SFAD[{p, m^2}]\text{True}
\frac{1}{(p^2-m^2+i \eta )}
$FCShowIEta = False
SFAD[{p, m^2}]\text{False}
\frac{1}{(p^2-m^2)}
$FCShowIEta = True\text{True}