I’m on the fence on this one. For example here in the UK many keyboards including my own has the ±, so we don’t need the conversion.
It should probably come down to whichever is the more common notation. The instance in the Google+ Exporter topic was probably the first time I’ve seen it arise here, it’s going to be even less common after April.
It probably makes more sense to keep the conversion and educate users on the alternative for the rare edge cases where +- is intentional.