It depends whay you want from the "dummy".
If it's meant to improve uniformity in a multifinger device
wired as a mirror or diff pair, then the "dummies" should
be the endmost fingers of that same active area.
If it's more of a "spare" and not particularly matched to
anything, then you can put it anywhere.
If it's supposed to allow you to strap on 2X the gm, then
it's probably meant to match Mr. 1X and may want to share
the same active (and then both of those might get outboard
dead fingers, so that -if- it's employed, it's already set up to
match as well as possible.