This is just my opinion, but I think detect_indentation should always be turned on. The reason being that having mixed indentation in a file is worse than having the "wrong" indentation.
I use tabs strictly in my projects, but often times I work on websites built by other people and there's a hodgepodge of 2 spaces, 4 spaces, and tabs. It's best to leave it as it is and get out as soon as possible.
