depends on the shifter. some have trim, others do not.

DT and bar-end friction shifters have infinite trim.
shimano road STI, depending on model, have trim where it allows you to have another indexed position between the 3 indexes. (some of the low end STI levers do not have it.)
campagnolo escape does not have trim
campagnolo 2009 levers have a lot of indexed positions giving you plenty of trim.