Gravity is not the factor here, light is. Root tips avoid light. More specifically, they produce auxins that trigger light avoidance/growth inhibition reactions. That's why you have curly tips with the PTM. One of several reasons why I haven't used PTM in 20 years.
Put them in some soil. Plant point up, and the root tip will break out, grow up, U-turn and head down, lifting the seedhead up through the soil point up.
Plant point down, and root tip will break out, make small up and U-turn down, creating the elbow that will grow up through the soil, lifting the seedhead up through, point up again.
Plant tip sideways, root tip breaks, makes horizontal then bends down, creating elbow to lift the seedhead up through the soil, point up once again.
See the pattern? Seeds don't grow straight root tips no matter how they are planted - but letting them sit around in paper towels when they are in their crucial seed-break orientation stage does waste vital energy and induce unnecessary stress.
The seed knows how to grow. Give it what it needs and let it. -b420