As Jack said, proper quartering of the crankpins is what sets the "rhythm" of the exhaust. If the quartering is out, the exhausts will not be evenly spaced. The "rhythm" won't be even and steady, like this scene with WM 734:
The other aspect to setting the valves is correctly positioning the valves in the fore and aft direction within the valve cylinder. This is what Steve describes in the post above this one. In this case the exhaust may be perfectly timed and square, but one exhaust beat will be noticeably louder or softer than the others.