if boiler has a pump overrun, or you've got trvs on all the rads, the by-pass will need to be open a little, if not it can be closed
I'm guessing that by pump overrun you mean that when the thermostat turns the boiler off, the pump keeps running ? If that's right then no, it doesn't have that facility & I have no TRV's on any of the radiators.
So I'm definitely OK leaving this valve shut then ? ( Can't you tell I ain't too happy with anything gas
)