NOT EXISTS Versus NOT IN

I have two tables :

tableA
a_id (int)
value (varchar 255)

tableB
id (int)
a_id (int)
b_id (int)
c_id (int)
d_id (int)

Both these tables contain considerable amounts of rows, but over time tableA will end up containing orphaned values (i.e. the a_id is not used in tableB) and this problem cannot be rectified by setting, for example, cascade deletes.

To fix this problem I decided to write a simple stored procedure to purge all values in tableA where its a_id is not used in tableB :

DELETE FROM tableA WHERE a_id NOT IN (SELECT a_id FROM tableB)

Now although the following document relates to postgres :

http://archives.postgresql.org/pgsql-sql/2003-12/msg00174.php

I was interested to find out if I should be wary of using NOT IN in my query.