It can have a great effect. Using probe vs. all out assault may mean the difference between living and dying. You must learn each one and what it does (hint all out/hold attack or at all costs means - no stopping till most everyone's dead). Mixing ROEs is OK, but if 1 unit retreats, it can make all units retreat (like Bull Run, where panic in 1 unit caused McDowell's entire army to skedadle).
Remember - The beatings will continue until morale improves.