For prevention of testicular atrophy, Bill Roberts advocates daily injections of HCG at 500-1000 ius per day during the cycle.
I don't know the extent to which this will keep you producing natural testosterone when you are using lots of exogenous androgens (if at all), but if it will, it would seem that gains would be better than they would without the HCG.
As mentioned in the other posts, HCG use at the end of a cycle is counterproductive since it will interfere with HPTA recovery.