Well this really depends on what you are doing with your plants also. First of all,
what are your plans? Are they going outdoors when the weather permits? How long
are you planning on vegetating them? Or is this an indoor grow?
Personally I keep my flouro tubes about a half inch on top of them
to minimize stretch at all costs. You want a short stocky plant, you dont
want a stretchy flemsy plant that cant withhold the weather. Or if your
planning on going indoor you dont want a weak stemmed plant that will
be hard to stand on its own without support when it comes to supporting
buds.
Please let me know your plans a bit more in depth and I will help you out
with whatever I can. Happy trails my friend!