Um, my engineering and horticultural background both say otherwise. Photosynthesis DOES discriminate by distance... and VERY rapidly if you are using a single indoor source of light. Light intensity drops by the square of the distance; meaning the light 2 feet from a light source has 1/2^2 or 1/4 the light intensity of the light one foot from the light source. The same light 3 feet away has 1/3^2 or 1/9 the light intensity at one foot, and so on. This is a huge problem when using single source indoor lights. See this Nasa image for the fall off of light intensity with distance:Nice write-up!
Just thought I'd chime in with a caveat (there may be others) to this:
Photosynthesis does not discriminate by distance, but rather it obides by light intensity and here is a case where strapping a seedling around a lamp makes no difference whatsover.
Compare this to the sun: the sun is so far away that the light intensity at soil level is virtually the same as light 100 feet closer to the sun. So when growing weed in sunlight there is no measurable difference in intensity between top leaves and lower leaves (unless they are shaded). The square of the distance rule does not really apply (it is so small as to be unmeasurable), and hence outdoors your statement is correct. But indoors, it is not. You are better off having your plants hug the light as close as they can without burning to get the most energy from the bulbs. I put my cool floros right above the leaves by an inch or so. The MH I set up about a foot above the tops to keep the leaves from burning.