Mo:
If you feel your using too much time in fit up then the joints cut that need extra work during fit up aren't being cut right in the first place.
Making sloppy joints to save time during fit up isn't the answer. Making correct joints in the first place is. If the joint is cut correctly then fit up shouldn't take a huge amount of time.
As far as your exact question about a beam with hidden shoulders, my first thought was the possibility of some rotation, although very little, I would imagine. But also, load goes to stiffness, if a joint isn't stiff then the load shifts to where it can and this could put added stress or load to another area.
Of course this is my opinion.
