I have a question about proper running ductwork in attic.

I have had someone tell me best to have ductwork in an attic next to and
even run between ceiling joist when possible and have the installation blown
in on top instead of having the ductwork hanging from several feet above the
joists. Said that even with insulation wrapped around the ductwork, the
heat/cold in the attic will eventually seep into the duct and will get a
blast of unheated/hot air out when first comes on. By having the duct as
close to the living space when possible and insulating as much on top of the
ductwork keeps the heat/cold loss to a minimum.

I can see where this may make sense, But is this done in any real
installations or would be praticle. I have seen somewhat both ways of being
done, not so sure if that was intended when I have seen the ductwork laying
between or across ceiling joists instead of being hung a few feet above ?

Any ideas as to do/not do it that way ?

Thanks,
MC