I do it a few times as I'm hardening off the cuttings, at the point where they first go into an actual pot of soil. I water with only plain tap water at this time, but give a light, weak, fert mist a couple times during the hardening-off period. You don't HAVE to do this by any means, it's just a nice little boost that I have found helpful in my own cloning environment, which is now in the side light from a big halide, and as soon as they are rooted, my clones have been TAKING OFF and looking hungry early.
I would say start a reglar nute program when you see that teh clones have about doubled in size from when you cut them. That's nice thing about clipping the leaves- when you see 3 sets of leaves that have no evidence of being clipped, you're good to go- feed, flower, whatever.

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