Rough lumber isn’t just coarse to the touch; it’s also twisted, cupped and bowed to varying extents. These changes in shape are just what happen to wood as it dries after sawing. And if this isn’t enough, rough lumber also shows a surprising variation in width and thickness from board to board because sawmills aren’t exactly precision instruments.
Jointing and planing is all about making boards flat, edges square and surfaces smooth. You’ll do yourself a favour by cutting project parts to rough size before milling. The shorter a board is, the less material has to be removed to make it true, all things being equal. That said, never try to joint and plane lumber shorter than 12″ for safety reasons.