It seems to me that if you leave a small gap between your tennon and mortise floor, after that small gap fills up it will not be any different than if there was no gap and further smoking would still push the stem up. To be honest though, I don't think this would actually happen.
Now I am not a serious pipe maker. I just make stuff for me and some friends and a few gifts. But this tendancy to nit pick one specific thing is not unique to pipe buyers. I have been in the market for a nice digital SLR camera. I am finally trying to make the switch from film since you can't buy any nice film anymore. I have noticed that people obsess over the littlest things. There is always some little detail that they are convinced is the make or break issue. For these cameras the issue is the number of pictures taken. If it is too high on a used camera, they think that it is no good. I have seen this same issue in other consumer products and I think that pipes are no different. I think it is more annoying in a hand crafted item like a pipe because in essance the over critical buyer is saying,"This is a piece of crap and you don't know what you are doing!" In that case I would say,"Get out of here and make your own pipe if you are so smart!" I think there are enough sane people in the pipe community that you will not have trouble selling your pipes if they are well made in the areas that the respected masters here on this forum say are the real make or break issues.