The default is red arrows on DL & DR. I don't use the formation a lot myself, just face it a lot. Other forum members seem to like red forward arrows on AML, AMR, sometimes ST (I don't see the point on ST), and blue defense arrow on all four defenders (again, I don't see the point).
Having red forward arrows on DL & DR would mean you intend on attacking heavy down the flanks and believe your two DC can handle most of the defensive load.
Personally, I'd try running red forward arrows on the outer MCs like so:
DL--DC----DC--DR
----------------
----MC-MC-MC----
AML----------AMR
-------ST-------
Just my opinion, don't be afraid to experiment