When is a crack wide enough for concern? It is not so much the width of a crack that describes the severity of foundation movement as is the floor level variation. I have seen many buildings that have crack free foundations, but are tilted by several inches. This can occur when the floor slabs and foundations are heavily reinforced (or when post-tension foundations are used). This foundation is obviously under distress due to settlement or expansion even though there are no cracks.
On the other hand, I have seen buildings with foundations that are relatively level (within 3/4″), but have a number of cracks, large and small. Cracks can also occur with inadequate construction workmanship that has nothing to do with soil movement under the foundation.