Fun fact: “cis” and “trans” both began life as Latin words describing location in relation to something else. In Roman writings you’ll find references to Cisalpine Gaul (“Gaul on the side of the Alps nearest us”) and Transalpine Gaul (“Gaul across the Alps on the other side”).
So really, when you say “cisgender” or “transgender”, you’re saying something like “gender identity that falls on the normative side of the spectrum” and “gender identity opposite the normative side of the spectrum”.
And possibly “gender expression” is a better term than “gender identity”. Semantics, man. Language is such a tricky thing.
Anyway. Identify people the way they want to be identified. It doesn’t hurt you and it’s a little kindness to the other person.