There is a roundtripping support between VS2010 and VS2012 ie you can move back and forth between the two versions without any issues. However prior to VS2010 most projects needed to be converted/migrated to the newer version of VS and you couldn't go back to using the migrated project in the older version of VS.
In short you would not be able to keep developing on the same project in both VS2005 and VS2012 without creating two independent project files. That I do NOT recommend since it means you will be a maintain them to be in sync.