I would never suggest leaving a exposed mortise with a 16th on either side, nor would I leave excess room above and below the tenon in a frame, however a tie beam is wider than a two inch tenon and the sides of the mortise will not be seen, don't let my question convolute my thinking on precision, Precision is the way to go or you will pay the price ten fold down the line. That being said, again what is the problem in a hypothetical bent where all critical points are on then noses and bearing points not the side faces where plates, joists and purlins leave that tie beam in the right place?

I pose this question because most people who work in a shop do not all have the same degree of precision, so.. if the shop takes the attitude of having a more narrow tenon than fit up will proceed more efficient. To some up long winded post to explain reasoning I still ask the question.

Are there any problems with having room on each side of the tenon in a mortise (when it is concealed, therefore disregarding the aesthetic debate)?