I may be shootin out my ass here, but I think there is a big difference between "Cleared approach" and a cruise clearance when still on a victor airway or even a STAR. With a "cleared approach" clearance, if you are not on a published segment or route that is published on the approach plate then you can not vacate your last assigned altitude without telling ATC, whereas in a cruise clearance you can descend or climb within the assigned block of altitude without notifying ATC. Now... from my understanding, once you are on a published segment or route depicted on your approach plate and ATC says cleared approach, then you can descend without advising ATC. I think there is a difference, and if your'e not careful, it could bite ya in the butt. If anyone can verify or rebuff what I just stated it would be much appreciated.