I've been testing all lenses at 25x and 50x focal length. In real life shooting, it may not make too much difference if you are within +/- 10.
If you mean within +/- 10x of focal length distance, probably true.
If you mean within +/- 10 AFMA units, I'd expect that to make a difference in many situations, either shooting wide open or with close subjects. One unit is 1/8 the depth of focus at max aperture, so a 10-unit difference is substantial. A 1-2 unit difference can be noticeable with an f/1.2 or f/1.4 prime, on average, but any given shot will vary a little too, probably in a similar range. A 3-4 unit difference with an f/4 or f/5.6 lens is probably not too noticeable.