Normally, a slatted base is enough to support the weight of your mattress.
However, if the slats are old, bowed or damaged, your mattress won’t be getting the support it needs. If that’s the case, place a bunkie board on top for extra reinforcement.
One thing to remember is that bunkie boards can be about 3 inches thick. Make sure this increase in height won’t affect your ability to get in and out of bed, particularly if you suffer from back pain or mobility issues!