Believe me in my 26 years of air traffic control this has been discussed so many times .......If you really want a good idea on how big a deal it is read this
http://www.faa.gov/about/office_org/...007-01-274.pdf
But that being said every good controller I work with will hardly ever give a new altitude assignment if they want the aircraft to meet previous restrictions. We just wait until that happens and then reclear the AC. Much simpilier and no issues to explain.
But when I have my pilots hat on I always put the responsiability back on the controller by using a readback that states what I will do. If not corrected I have a good position to stand on if something comes up.