ORA-01555 while export

I have a 1 terabyte db,

we do export every night for crucial tables.

so we are getting ORA-31693, ORA-02354, ORA-01555

Currently undo retention is 1000, I've changed it to 6000 and move the export time for night to late night.

select max(maxquerylen) from v$UNDOSTAT;

MAX(MAXQUERYLEN)

----------------

           33321

select max(maxquerylen) from DBA_HIST_UNDOSTAT;

MAX(MAXQUERYLEN)

----------------

           38500

but i felt like the db will ask for more.

the questions:

1) why some of the solution at oracle support suggesting to change this value to something big before export, and change it back to normal after export. is there something not nice if the value stay big?