It can depend on the fiber and the size of the item. A cotton sweater is heavy and oft times will grow from the weight. I tend to measure such items hanging. But if acrylic or blend, you’re usually apt to measure on the flat.
If you’re doing pieces of a garment, best you rely on the no of rows as that’ll guarantee same length as your knitting gauge may change (bit looser, tighter on some days). If I’m not working the body in the round, I usually work all body pieces at the same time on a circ so my incs, decs, lengths all match.