How much light does CNTL flash "spill" into scene?

I sometimes use on-board flash (A77) to trigger my F58's for portraits at weddings, and up close, you can find the control flash adds a small unwanted extra catchlight to the subjects eyes, but I just put my hand in front of the on-board flash, it still bounces off the ceiling and triggers the slaves, but no unwanted catchlight...

Although these days, I tend to use cheap radio triggers and manual set the flashes for exactly the conditions I want.. Or I bounce the control flash and use that as soft fill light, and use the F58's ratio control to get the more controlled lighting I want..