Quite obviously it makes no difference, unless you have some super weird special corner case where your PCB has to act as a shield, i.e., the PCB is doubling as one of the walls of the box, then you would want the outside grounded.
I have found quite a lot of statements like the following, but nowhere an explanation of why it should be on the bottom.
As a rule of thumb, simply ignore all "advice" which seemingly makes no sense and comes with no explanation whatsoever. Illogical statements require strong evidence and clear explanations.
Besides, I don't think there is common convention. It can be on either side, both styles can be widely seen in use.