The ISTD book (ed. 1983) says: "every step should be taken on the ball of the foot with the knee flexed". Elisabeth Romain in "Let's go dancing" confirms this.
I would say that Walter Laird is a bit more "prudent" in only describing this foot and leg action for the first two steps of the basic (the back - replace steps).
Now for the 2nd step of the jive chassé, this short "a" count, is this flexed knee technically/esthetically a must/desirable/feasible ... ? I tend to close a more or less straight leg, certainly at quick tempi.
Thanks for the advice.
Erik