Valve system with rotary stepping actuator to actuate a valve stem. An actuator drive shaft is supported for stepwise rotation in clockwise and counterclockwise directions to impart rotation to a valve stem nut to axially move a valve stem to open and close valve flow trim. The drive shaft forms peripheral first and second slotted circular drive paths co-axially spaced from each other. The drive paths include outwardly-opening, circumferentially-spaced slots which are matched in number, size and spacing, but the slots in the first drive path are rotationally offset from the slots in the second drive path by a set fraction, for example one half of the slot spacing. Actuation assemblies provide separate forward and reverse actuation cycles to the drive shaft to incrementally rotate the drive shaft in predetermined angular increments (steps) set by the slot spacing. Due to the offset, a counter step in a direction counter to the direction of a directly preceding step rotates the drive shaft by an amount less than the predetermined angular increment, as determined by the set fraction. If the set fraction is one half, the counter step rotates the drive shaft by a half step, while the next following step in the same counter direction is a full step in the predetermined angular increment.