I looked at the link so graciously provided by MiaCaro, and a few others I found on my own, but none talked about whether or not the mouth is actually, physically washed prior to autopsy. To me, it makes no sense to do so. I mean, that might compromise other evidence.
I remember when the one body was stolen from the lab, beer had been poured down the throat of the dead body. So, it would seem to me that if one cleaned the mouth with water as one washes the rest of the body, that might compromise stomach contents, etc.
Does anyone know, maybe, a forensic text I might find that could offer help? Thanks!