The heat from combustion that an aluminum head sees on one side causes stresses that the other side of the head does not have. This why an aluminum head is almost always warped after it is removed form the engine.
This means that the cam journals are no longer in line with each other. If the head is milled in this condition and then bolted back on the engine, the camshaft will be in a bind while running. This will usually break the camshaft or seize the camshaft to its journals, causing bent valves and broken guides.
We Straighten our heads in our WRE Head Straighten Center. We align the cam journals back to OEM Specifications before milling the head or machining the valve job.