A cardiac rhythm management system includes atrial shock timing optimization. Because an atrial tachyarrhythmia, such as atrial fibrillation typically causes significant variability in the ventricular heart rate, resulting in potentially proarrhythmic conditions. The system avoids delivering atrial cardioversion/defibrillation therapy during potentially proarrhythmic conditions because doing so could result in dangerous ventricular arrhythmias. Using Ventricular Rate Regularization ("VRR") techniques, the system actively stabilizes the ventricular heart rate to obtain less potentially proarrhythmic conditions for delivering the atrial tachyarrhythmia therapy. The intrinsic ventricular heart rate is stabilized at a variable VRR-indicated rate, computed using an infinite impulse response (IIR) filter, and based on the underlying intrinsic ventricular heart rate. The system withholds delivery of atrial cardioversion/defibrillation therapy until the intervals between ventricular beats ("V--V intervals") meet certain criteria that decrease the chance that the atrial cardioversion/defibrillation therapy will induce a ventricular arrhythmia.