To mix D-76 I just use my hard acidic (Ph of 5) water at the temperature that comes out of the tap and it works fine for me. I just fill the jug about 3/4 of the way, add powder, then top off with water to one gallon [3.8 liters], shake for a few minuets, let stand overnight and if there is any powder left in the morning I shake it again. This has worked every time for every powered chemistry I have made.
I would avoid using boiling water because you could burn yourself, and really it isn't worth it just to make up some D-76, it isn't like the Kodak recommended mix temperature is a magic number that will change the way the D-76 works, it is just the warmest they want you hearing it to make it dissolve easily, exceed that temperature and you may find out that there is a magic temperature where D-67 can be destroyed.