Flower them pretty soon if you have a height restriction- even if there is like another 3 feet before you acutally HIT the light, you get heat burn well below that.
A good SOG depends upon growing from clones, but if you are doing it with seed plants, flower them as soon as you see the nodes go alternating, and arrange them like this:
-Put the tall ones around the edges of the room, and the short ones under the light's hot spots
-Strip any scruffy, thin growth that gets no light, up to about 3" off the top of the soil, and remove any side branches that are lanky and pathetic. Leave 4-5 very strong leads per square foot.
-Never cut or remove the tallest branch on the plant. This turns into your main cola