I'm not Cathe either, but it is correct that doing step daily, or indeed doing ANY mode daily to the exclusion of others, puts you at greater risk for overuse injuries both for the joints involved and the working muscles (and their connective tissues) involved. Stretching probably would not reduce that risk very much.
It's always a good idea to have as broad a mix of modes of exercise as you can, in terms of degree of impact on the weight bearing joints, in terms of the ranges and planes of motion involved, and in terms of the muscles worked. This is true for the prevention of overuse injuries, AND to prevent the body from becoming too efficient at one particular mode. That's where Cathe's workouts are such a blessing, because she offers step, step-kickbox, pure kickbox, high / low, and the Boot Camp format.
For intermediate to high intensity and choreography complexity, I'd suggest getting the Cross-Train Express Series and the Timesaver DVD, because these offer a broad range of cardio and resistance methods onto three DVD's. Also consider the Circuit Max / Cardio Kicks DVD at this time for Cardio Kicks, which is a really good kickbox routine that you can grow into. Also the Step Blast / Step Jump Pump DVD for the Step Jump Pump workout; there is some good high / low aerobics that you can grow into as well.
HTH -
A-Jock