I'm sure there are multpile points where you can inject
an offset current / voltage to push the output CM sum,
but practicalities probably favor one.
For example, you probably do not need or want a high
V/V gain because the VCM generally needs only small
correction if the circuit is balanced and mismatch-
desensitized (large devices, etc.) and any excess gain
probably only increases noise coupling. A common-
source amplifier's gate would be a bad place to work
then, as gain is high.
Also the output of the common source stage is where
you pick up most of your gain (gm*Rout) and any
element you add will degrade Rout hence the -desired-
forward gain.
But nothing should stop you from trying out alternative
points of offset-injection, just to satisfy yourself that
you have in the end selected the best for the use.