subquery join order by

             Hello,

(I googled and read docs before sending this e-mail).

Is it necessary to use order by twice (inside and outside) to get the
proper order if I have an ordered subqery in a join?

select * from (select distinct on (b_id) * from a order by b_id, id) sub
left join b on b.id = sub.b_id;

or

select * from (select distinct on (b_id) * from a order by b_id, id) sub
left join b on b.id = sub.b_id order by b_id;


It seems to me that it's enough to use 'order by' only inside wheter 'by
desc' or 'by asc' (b_id), however I'd like to be sure.

Thank you.

             Mage