from my experience, if you are dealing with seedlings (from seed), its more about the heat that causes issues, rather than the intensity from a light that is too close... (a distance within reason; I mean you cant put an air cooled 600 or 1000 only 3-4" away)
When the seedling first breaks soil, it will pretty much adjust to whatever light is being used:
example: if you throw a seed outside, it will be ok with the intense, direct light from the sun, and grow great all the way through. However If you start seeds under cfl's, they adjust and get used to the low intensity light, right from the get go. Thats why when you go from cfl to straight outside, or from Cfl to straight under 600 or 1000 lights, sometimes the bleach, or are shocked becasue they are not used to that much light....
in theese cases you would need to put the light higher and slowly lower it untill you have a balance between the distance of light to plant, and amount of heat that is at the top of your plant....u want it as close as possible without burning crisping ( too much heat) or bleaching ( light intensity too much) the leaves...
I have started seeds under my 1000 HPS and MH (it is air cooled) and I put it about 18- 20" above the seedsings. And if my lights were not air cooled, my plants would fry because its sooooo hot, not because its too much light.
And also, from my experience, clones is another story...
Usually clones shouldn't be put under 600 or 1000 lights at close distances, because most clones are relatively "new" they dont have good enough root structures to handle the intensity at close distances. But after a weeks or two under the lights, I can lower to about 18- 20" (air cooled hood)