Reconfigurability in versatile systems of modular robots is achieved by appropriate actuation of their modular units. Optimized distribution of active modules in the modular architecture can significantly reduce cost and energy in the reconfiguration tasks. This paper presents methodology for distribution planning in modular robots that results in minimum number of active modules, while guaranteeing their capability to reconfigure. We discuss the optimal distribution problem in layout-based and target-based planning, so that the modular robots can respond to instant commands for reconfiguration with an initial planar pattern or a target three dimensional configuration as input. We propose heuristic algorithms for the corresponding solution in different scenarios. Our approach is demonstrated by applying the algorithms to Mori, a modular origami robot. The simulation results show that the algorithms yields distribution schemes with high quality, thereby appropriate for real-time applications in modular robotic systems.