Stretching is always important, but even more so if you want to be able to do a squat or deadlift properly. Lack of flexibility in your ankles, calves and hamstrings can prevent you from doing these exercises correctly.
Cathe will not only work your muscles hard in the STS Meso #3 optional leg workout, but will also do a lot of stretching during the breaks and will encourage you to always join her for an extended stretch after the workout that is included with every DVD. Remember good flexibility is always a prerequisite for good form.