To be honest, I think it's irrelevant. I'm sure the thought has crossed all of their minds eventually (Rachel could have used it to prevent Tobias' nothlitism, Jake could use it to save Tom, Cassie would probably just go back and tell herself to skip the mall that night, and Marco probably would have used it to test his dating skills [ask a girl out, and reverse it if and when he got rejected]). But I doubt that they would have been "allowed" to change time willy-nilly. If the Visser wasn't allowed, the Animorphs definitely wouldn't have been pardoned; esecially with Crayak and the Ellimist watching their every move--really, they have nothing better to do with their time?
Sure, they could have prevented David from joining the group. They could have done a whole lot of things. But I think the Ellimist would have shown them the consequences of their actions and reverse time to the moment they decided to use the Time Matrix.
My question, though, would be this: If the Ellimist and Crayak are manipulating--or "not manipulating, as they would have us believe-- the lives of the Animorphs (and we assume that everything, in the Animorphs universe, is causally determined, then what was the point of David' finding the blue box in the first place? To teach the Animorphs to hide their important plot devices in better places? To show us that new Animorphs simply wouldn't work?
Well, that doesn't work, since they ADD NEW ANIMORPHS LIKE THIRTY BOOKS LATER!