It is my understanding that I can not save a 2012 .rvt file down to a 2010 .rvt file for one of our clients. How are multiple disciplinsusing different versions of Revit getting around this?? 

They should be on the same version. You can use IFC to communicate Revit data (with some loss) between versions.

If They Only have 2010 then you should keep your old version of the 2010 and not open any of their stuff with your version.  You can still use PDF and like methods to show things but the versions are not compatible.  Once opened and saved in a newer version It will never be opened in the older version.  One way for them to say you have to update every year.  My firm is using 2013 and we cannot do anything with anyone around my area yet as everyone here is still in 2010 or 2011 or even still in acad.