I think it's preference. Some people have the helper go away as soon as they pick up the handle. It's legal as long as one person has ahold of either end of the lines. I think folks wait for power to come up because then the takeoff thrust is always the same, making takeoffs more predictable than having RPM ramp up. Because electrics reach full RPM on the ground, the torque can make takeoffs sporty with either prop rotation direction. I take off with constant, reduced RPM, then have it go to full power after a lap or so.