Hi Colin,
The ultimate type of housing is I suppose the use of lap joinery of which many examples exist in Europe, that said I think that you are really referring to the housings cut using typical American sqaure rule and in particular on the end of beams and braces.
Where a beam is expected to carry floor loads then it is usual and good practice in scribe rule carpentry to cut a shoulder with a tapered haunch. This ensures that the full section of the beam is available to take shear loads rather than just the tenon. This taper shoulder will also tend to pull the joint up very tight against the bearing face and mortice housing face.
On many old buildings read 1600's and earlier) you will not necessarily see these sloped haunches and this is simply because the frame was not originally cut with it in mind that there would be upper floors. These "open halls" were later ceiled over and now those beam end connection joints are not strictly fit for purpose but if the beam is also supported by a load of wall studs or other type of solid infill walling underneath the beam then the lack of a tapered haunch probably makes little difference. Failure and withdrawl of this type of joint can be seen after the passage of many centuries but rarely in itself will promote a collapse.
Regards
Ken Hume
http://www.kfhume.freeserve.co.uk