[olefebvre]

It's most common to encounter Me in Mtt documents especially those designed for export purposes like the translated flight manuals. Those are definitely using Me rather than Bf.
The documents designed by Mtt for the RLM feature "Bf" rather than "Me" (except in preliminary versions) but internal Mtt documents use both and even the RLM internal docs use both... sometimes within the same document.

I would probably follow Mtt way of thinking and call exports 109, "Me 109" but technically both designations are valid.