Fast answer
A useful white-town day needs restraint. Pair Locorotondo with Cisternino for the compact version, use Martina Franca when the day needs a stronger evening, and add Ostuni only when the route has enough time for an outer stop. The goal is not to collect every white town; it is to keep walking, parking, lunch, dinner, heat, and return timing from turning the valley into a rushed checklist.
If you only do one thing
For a first white-town day, pair Locorotondo and Cisternino, then decide whether Martina Franca should hold dinner. Add Ostuni only if it replaces something rather than stretching the day.