The top driver is the full range driver, so yes center on that one. The bottom driver only goes up to 200 Hz, so behaves pretty much like a subwoofer (which cannot be located by ear).
Also keep in mind that even big/heavy monitors on stands can get tipped over. Years ago my 6 pound cat (completely out of character) came running in, took a leap off the front of the couch, and bounced off a 10" x 10" x 20" monitor that was on a 12" high stand, knocking it over and breaking one of the binding posts.