If shapes ever become unaligned from the grid, the Re-Align Shapes tool will nudge each shape to the closest gridline.
Note: If the flowchart contains a connector that has one end "connected" to a shape and the other end is not, the connected shape will not be re-aligned. One quirk of Excel is that moving a shape with a free floating connector can cause the connector location to be changed in an unpredictable manner, so these shapes are left as-is.