We see a majority of backfalls in our dojo from the kihon type throw. What is interesting, though, is how many newer students ask if their fall should begin/use their inside leg or their outside leg... .
I find that I will often pivot over the outside leg at nominal speed, but if the throw is coming hard & fast I will usually pivot over the right leg, no matter which side the technique is on. sometimes resulting in nage getting a nice whack on the instep if they're not out of the way when I hit the mat -
- for which I always apologize.
I encourage new students to try back/side falls over each leg to see which side feels more natural to them.
Like many things on the mat, everyone is a little different as to what will work best for their bodies.