Most Roy combos work better with percent tacked on. Easiest example to see is jab -> jab. At 0 it doesn't work on a lot of characters, but it will almost always work if 3 or 4 percent is already on them. This is because at higher knockback, you are stuck in the 'hit' animation for a longer period of time, effectively making the window to combo larger because the opponent won't be able to act out of the hit frames before you can throw out another attack.
As for fthrow nair nair, it works reliably even on fast fallers at mid percent. You could conceivably string 3 nairs on a fastfaller if the stars align.