Hm-mh.

My own, very informal testing, has never shown much of a difference between surface and 6""+ down. But, if the surface is covered in goop, including suntan lotion slime, I always go deeper. If you want to go a step further, it would be interesting to know if there actually is a detectable difference between surface and immediate sub-surface.

One other note: on pools under full sun you would expect to see a difference in chlorine levels between the surface and 12" down, if the CYA level is zero. Half-life at the surface can be as little as 15 minutes.