We have always used File -> Save -> Save Precise Assembly to store our precise assemblies in NX, (running it Teamcenter).
In NX12 there is no "Save Precise Assembly" menu choice under File -> Save, where can I find it?
In NX12 there is only Save, not as previously, both "Save" and "Save Precise Assembly".
Now, when you want to store an assembly as a precise structure you go to the Assembly Navigator and right click on the assembly node and chose "Set Precise on Save"
Having done that, look at the new Assembly Navigator column "Precise Structure" it will indicate upcoming save action with the "Precise State Modified" icon
When you later on save the assembly the icon in the Assembly Navigator column "Precise Structure" change to the "Precise" symbol.
Notes and References
Teamcenter Integration for NX -> Basic creation and management of data -> Saving precise assemblies -> Creating and viewing precise assemblies
OS Version: 1064