There are two trains of thought on this.
Keep sparging until you get your proboil, maintaining your 1" water on top. Pro: keeps grainbed fluid and compacts the lower section less. Con: Uses more water that you throw away.
Calculate sparge volume so that you reach preboil volume just before running completely dry. Pro: uses less water Con: Could be less efficient if you miss calculate requiring a water topup later. Grainbed can collapse and compress halting runoff.
I tend to compromise. I calculate so that I'll leave about 2 gallons in the tun. That means the top half does dry out, but the area near the false bottom is still relatively fluid.