Most guys drop em at least 6" before a notch is even being thought of.
If they're trying to sell you on a notch to go with 4" drop springs.... It sounds like they don't have a lot of experience with these trucks...or they're just trying to pad your bill.
Here's the write up I did on the last truck that we dropped and notched.
http://www.67-72chevytrucks.com/vboa...ight=worthless