I know I'm not the best Falcon around to answer this, but for me, the general idea with grabbing fastfallers is upthrow when you want to give yourself time to get into place for techchasing or predicting stomps/knees in pretty much any direction, whereas with dthrow, you generally mix it in if you expect the flubbed tech, or the tech in place or towards you. Or dropzone, if you're at the ledge, lol.
I dunno, there really isn't any clear cut distinction between when to uthrow or dthrow. You pretty much have to learn and feel it out per person. I do know, however, that at 80% to about 100%, you uthrow re-grab, or tilt on spacies if they don't DI away far enough. And at about 100% to 110%, you can fsmash them if they don't DI away or up. It's some other, lower percents for Falcon dittos, which I do not know as concretely.
My general rule of thumb is to regrab for the first techchase, than try to predict a stomp/knee for the next based on how they teched the first one. But, once again, it depends on how things shake out, ha ha.