This is very true. The OP's question is loaded to start with, is states "a casting that is free from internal stress", good luck finding that animal. Castings, by their nature are full of internal stress. The outside cools and solidifies first, leaving the inside to cool, and shrink(or at least want to) after the outside is solid. This causes tension within the casting, the inside is pulling in on the outer surfaces. As the cooling continues, the part reaches a point of equilibrium and maintains it's shape, until, material is removed from one side and the state of equilibrium is upset. The material in the center no longer has anything to pull against on that side, causing the casting to go convex on the machined side.
Stress relieving will help remove a lot of the major stresses, but some will always remain. This is what causes a part to change shape.
Various types of machining can induce stress into a part, especially the use of dull cutters and high pressure situations. This can have a burnishing effect and impart a compression stress into the very outer surface of the material. This effect is usually eliminated in taking light finish cuts with a sharp tool of free cutting grinding wheel.
Best approach to machining a casting is to rough it out, at least cleaning all surfaces which require finishing, and leaving as little material as possible. This should be followed by stress relieving, then finishing.
