It's a tough one. I chose not to use the "Count Du Money" code in MM4 because there is no way to discover that code in the game, except by inspecting and decoding the game files to find it. It feels more like a developer's QA tool than a legitimate code.

The way I view it, if that location is legitimately accessible within the game and is not obviously a cheat / QA testing tool, then it's fair game. This is highly subjective though and other people likely have alternative opinions.

My favourite example of a cheat / QA test room is the Ultima 7 "behind the blacksmith's chimney" teleporter. You're obviously not meant to go there and it has copies of most of the important plot/quest items in it.