Good point. It probably doesn't even make any difference in most situations.
However, as far as having wiggle room at assembly time goes. If you've got room to move it around to where you want it then there's also room for things to move out of place while you are getting clamps on. I'd rather start with it too tight and get it so it can only fit correctly aligned during dry fit. Otherwise it will never go back together exactly the same once you've got glue on the sucker.
I did see Norm doing what the OP referred to, and if I remember correctly he didn't square off the mortice or round the tenon, so it probably looked like there was more room for it to move than there was. I can understand that since the episode was about how to use a router not how to make a M&T that he might skip rounding the corners just to get on with it. Every other time I've seen him do it he either squared the mortice or rounded the tenon, and I've never seen him say anything about leaving space before either.
-Leuf