I believe that is a very old thought process.  Unless you have "directional"
tires I don't think modern tires are effected in the least... I vote for
following the manufactures recommendation.  I do, and have never had a
catastrophic tire failure.

Radials should stay on the same side regardless of whether they are
directional or not for wear and balance reasons.
They will not magically exlode if you do not follow this rule.

<Controversial content>

Tire rotation is voodoo maintenance.  There is no actual economy in it.
 It does not extend the life of the tires.  It will only allow all of
the tires to wear out at the same rate so that replacement can be done
in full sets.  If that is important to you for some reason and/or you
can get it done free (or do it yourself) then knock yourself out.  If
you are paying for it then you are just wasting your money.

The alternative is to leave the wheels where they are and when one axle
becomes worn replace the worn tires only.  If they are wearing unevenly
(you will see this more quickly if you do *not* rotate them) then get
the suspension or steering problem causing the wear fixed because that
*will* extend the life of your tires.

</Controversial content>

-Fred W

Your mechanic friend is living in the past.
Today's radials (unless they are designed as directional - and few
are) can be rotated any way you like.
The first radials would take a 'set' when first driven, and needed to
roatae in that direction from then on, or the plies would tend to
seperate. Not any more.
As for Jim's balance remarks, I've never seen anything that would
confirm that.
One thing that's often overlooked is that, when rotating your tires,
if there's uneven wear on them, it's time to get that taken care of;
rotsating the tires will not fix that problem. I'm amazed at the
number of people who will look at their tires, see uneven wear, and
comment, "Well, I guess it's time to get the tires rotated."
Like that's gonna help!