For me, it depends on the keyboard. I am typing on my Macally MK96 right now, and I have to have it flat. When I type on an AT101W or my Filco, I have to have them at a tilt. In those cases, it seems to have to do with how much pressure I have to put on the keys. My Model M also lays flat, to support that theory.
Also, when talking about ergo, I am pretty sure that you should start from a correct typing technique. Of course, people can use whatever works for them, but there really is a best practice.