Standard practice when working to a line with a chisel is to start a little away from the line, normally about 1/16 or 1/32. If you think about a wedge being forced into wood you either have to have a place for the wood to go or you compress the wood. In a typical cross cut at the baseline the wood at the back of the chisel gets compressed a little. If you start away from the line when removing the bulk of the waste you allow for the movement of the chisel. Later light parings to clean it up can be done right to the knife line because the thin waste now has somewhere to go. Some folks shave the baseline square to the stock face and some choose to undercut it a little, from both sides toward the stock middle, to ensure there is no interference with the mating piece. Undercutting does not significantly affect the strength of the joint.