Hi,
Rooting flat is a call for drowning the cutting IMO. But, I may overwater my cuttings all the time ... They tend to take all the water next to them and rot.
To be safe I root cuttings pointing up with a slight angle, not totally straight.
Then whatever works best for you ... keep on doing it.
I have never had luck with burying and laying cuttings flat. They always rot on me.
Another drawback is that a cutting pointing up will make a bud and leaves at the top and those will have light to do photosynthesis from the start and produce energy for the cutting,
while a cutting that is buried will send white shoots that will reach the light and then start turning green. Sometimes they fail at becoming green and rot.
But I'm perhaps just unlucky when burying them ... I'll keep on making sure that they point up :) .
If I want more roots, I just bury more length of wood from the cutting.
Good luck !