If you only missed a few screws here and there, I would leave well enough alone. If you missed an entire area, then I would consider adding them even now.
Vince, just curious - did you lay the Hardibacker so the (2) 5 foot lengths went across the 10 foot room, or did you lay the 3 foot width across the room so you had to deal with the 1 foot piece?
Dodgeman