How strict on leaving early?

[Scrapper1]

This is just a volleyball 101 question. How strict should I be on a back row player who moves to the front row just as the serve is being struck. This happened quite a bit in the same match where I was overruled in the other thread.

Every time their setter was in the #1 position (while receiving), she would position herself just behind the #2 position player. Then right before the serve was struck, she would step in front to get to her setting position.

This happened every time and was noticeable to me. But I didn't call it because it didn't really seem to affect anything and I don't really know how strict I should be. It was a second or less before the actual serve.