The plane of focus is meant to be "flat" (technically it's always at least a tiny bit curved) .
The means that if you require critical focus, and you focus your subject in the center of the image, then recompose to move them toward one side, they've almost certainly fallen out of the point of critical focus.
I say "critical focus" rather than just "focus" because if you have a very broad depth of field, you probably won't notice much difference (if any). But if you have a shallow depth of field, it'll likely be a problem and using the focus point nearest to your subject (to minimum any need to focus & recompose) will get you better results (even if it is a little more time consuming to reselect the focus point.)