Rate of fall is certainly one thing that attracts bass and can trigger strikes, but bass don't always take the jig on the fall. When they prefer the bait on the bottom, being drug back to the boat, it is important to use a jig head heavy enough to maintain bottom contact and one that will transmit enough vibration so you can feel the bait coming into contact with rock, wood, grass and other cover. I usually try to use the lightest jig that I can still maintain a bottom contact with and get that good feedback from the jig.