Best practices for ensuring up to date sequences after table import

Note that 11g has support for sequence expressions directly in PL/SQL

Otherwise, from the Oracle Docs:

Beginning with Oracle Database 10g Release 1, logical I/O is not performed on the DUAL table when computing an expression that does not include the DUMMY column. This optimization is listed as FAST DUAL in the execution plan.

So any performance gain will be slight. In the following tests I dropped and recreated the table and sequence between each execution:

testbed:

create sequence s;
create table t(id integer);

tests:

declare
  n integer;
begin
  for i in 1..100000 loop
    select s.nextval into n from dual;
    insert into t(id) values(n);
  end loop;
end;
/
--average about 5s

declare
  n integer;
begin
  for i in 1..100000 loop
    insert into t(id) values(s.nextval) returning id into n;
  end loop;
end;
/
--average about 4s

begin
  for i in 1..100000 loop
    insert into t(id) values(s.nextval);
  end loop;
end;
/
--average about 3s

You should probably rerun these tests or ones like them in you own environment.

You usually don't want to disable parallelism as that will also disable it for admin tasks. Your best bet is to fix the queries that are causing the parallelism through adding or fixing indexes or through making full on schema changes.

Based on your updated questions...

Some people will change MAXDOP to 1 for vendor built applications because they can't control the database or schema and they don't want a single query to take over the entire system.

Personally I always keep MAXDOP at 0 except for some rare cases.

Parallelism is caused by a single operation within an execution plan having an execution cost which goes over a pre-set setting (the cost threshold for parallelism setting). When this happens the SQL Server will kick in parallelism so that it can multi-thread the request in an attempt to speed up the process. The default value for the cost threshold for parallelism is 5. In many OLTP platforms you'll want to raise that up to 30 or 40 so that parallelism only kicks in for the really expensive queries.