MacCready theory is perfectly valid for both thermal soaring and cloud-street dolphin flying as long as the assumptions under which the theory is valid hold. Since this is almost never, flying at an appropriate MacCready setting may not be optimum under any conditions!

Out west, you run the risk of flying into the ground when thermals are widely spaced and perhaps short-lived - hence the recommendation to slow as you get lower.

Under a solid cloud street, MacCready can give you good guidance overall, but goes out of the window if you have to fly fast in lift to stay out of cloud!

Mike