A germinating seedling does not receive light underground. Warmth, yes.
The point is, they'll still emerge regardless of light on or light off. A 68 degree soil temp or a 71 degree will still germinate the seed. Speed may be slightly different, but I've never seen it tested or felt the need to find out.
It's about as important an issue overall to growing the plant well as which method of cloning is fastest. Everyone likes to claim their cloning method is best (fastest) but no one has ever asked why cloning is a race. Why does your cutting have to show roots the fastest?
I clone in a cup of water. Roots take 10-14 days to show. I know this so I take cuttings that long before I need them. My room is then no less efficient than someone who clones in a $100 machine and has roots in 4 or 5 days. They just take cuttings when they need them.
I hope this makes sense. I'm kinda stoning out on my personal pot growing philosophy...