This may be another one of those "How do I approacht this?" issues.
In doing some assembly work with SW one thing that irritated me was just how easy it is to break mates when editing parts. It would be quite useful to have the ability to have more absolute mate references (or truly smart ones).
One example of this is a part that you'd want to mate with another by always using the highest surface in the Y direction. I don't think that there's a way to embed this kind of intelligence into parts at the moment. Having this degree of intelligence would allow you to edit and experiment with a part and not destroy a million mates in an assembly.
I haven't looked into the internals of Mates, however, on a superficial first-inspection they seem to be tied to the names of features, edges, points or planes rather than the feature itself. There seems to be little intelligence. Which means that, if you were to model exactly the same part without a single geometric difference except for the modeling sequence, a "Replace Component" would result in lots of errors.
It is far too easy to blow-up a complex assembly by making an innocent change in a part. The more I use it the more I understand why in-context relations and features regarded as problems. The sad part is that such abilities as defining in-context geometry is one of the big reasons many of use have looked towards the use of programs such as SW.
-Martin