I use a stopper that is 2 inches tall with a hole in the middle to prevent overflows (most of the time).
One of the recent second floor showers I tested in a recently renovated multi-unit dwelling (no I did not use the "C" word) overflowed due to the drain being plugged. So don't trust the center draining stopper to save you all the time.
I regularly test tiled showers and regularly find substantial leaks, maybe 10% of the time. I cannot imagine NOT testing a shower pan.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002