A very dense technical article from Dolby Labs dating from 1981 has been reprinted on in70mm.com.
http://www.in70mm.com/news/2015/mixing/index.htm

Here is an excerpt concerning ESB:

Usually only a few 70mm prints are made for any film, and they play in New York, Los Angeles, Chicago and other large markets.2 In these cases, the large majority of stereo prints will be in 35mm stereo optical; hence, the importance of monitoring through the matrix. In the spring of last year, however, Bill Varney found himself in a unique position during the mix of The Empire Strikes Back: "Ben Burtt [the supervising sound editor], the crew and I decided that since we were going out with over 125 70mm six-track prints as our initial release, the best thing to do was to make sure that those prints were going to work and work well, and to deal with the Dolby matrix later for the 35mm secondary release. So we monitored the film in a discrete format all the way and, once the six-track was done, went back and put it through the 4-2-4 monitoring. We were pleasantly surprised, and had to do very little in the way of correction; the picture moved into the Dolby Stereo mode easily.

“The two areas where you have to be careful, and that are hard to predict, are the boom channels — speakers two and four — and, more importantly, the surrounds. The idea was to make the film work on the front speakers without any surrounds or booms at all. We mixed the film by recording on just channel one, three, and five. We then went back and added the surround channel and the boom on the other tracks. The advantage to that is that when the film goes out in Spearfish, South Dakota, and the surround speakers and boom channels are not working, at least you know as mixers and creative people that the film is going to play and play well — with just the front speakers working.”