I agree with nosehair.
Descent planning is no where near the top of the list for most places.
It really hits home for the student when they get a PD descent, start down right away and go from 6.7/hr to 8.5/hr and lose 10kts across the ground. All I think is "hmm...that 10kts probably just bought them another .2 on the hobbs".
...and really it onliy takes about 10 minutes to teach it to a student, then they've got it forever. After one flight to teach descent planning, I find most of our students fly at 6-10k.
-mini