> By "heuristics" I don't strictly mean "rule-based", but you can do a good job in reducing the search space by a large amount if you program in some common sense that will discard obviously bad moves.

I believe that the fitness function used for evaluating build orders does this. It is very difficult to accurately evaluate how good a specific strategy is at a specific point in time, especially when you have imperfect information about your enemy. The code used to decide which build orders survive almost certainly used heuristics.