Personally, we have not seen a situation where imprecise assemblies were required. However, we have encountered customers who have insisted upon CMM supporting imprecise.
We strongly recommend running migration in a precise manner. Your released data should be precise, describing of exactly what item revisions were reviewed and released, and we suggest so should your migrated data.
Imprecise configuration or loading of your BOM revisions is done through selection of a revision rule. You may only unambiguously load the top level item revision dataset, but from that point south the item revisions, components, loaded will be selected using the active rule. The rule applies to the loading of your source CAD files, with the NX datasets being placed into the same item revision.
Precise loading may also be known by the ‘as saved’ condition. This can be especially true when you are migrating released item revisions, since they should not have been changed once a status was applied.
Again, why imprecise? Revisions can in fact meet form fit and function for items in an assembly, so loading imprecise does have merit when business process require it.