Cusp reduction can be considered in all of the following situations except:
The most important aspect in the evaluation of a suspicious cusp is the judgment of the amount of remaining dentin support. Cusp reduction should be considered when the outline form has extended half the distance from a primary groove to a cusp tip. Cusp reduction is strongly recommended when the outline form has extended two third the distance from an adjacent primary groove to the cusp tip.
The exception to reducing a cusp, where an extension has been two third from a primary groove toward the cusp tip, is when the operator judges that adequate cuspal strength (adequate dentin support) remains. Reduction of cusps occurs as early as possible in the preparation process so as to improve access and visibility for the operator.