Lucky Huggy's guarantee was not 100%
The book format is
Cleared to (destination) via (routeing) at (level), climb when instructed by radar to (level).
Level in terms of aircraft attitude is nothing to do with ATC or your clearance. Whether you fly it upside down, in a 45 degree bank, or any other attitude is of no relevance to ATC as long as you comply with the clearance. Rather we use
level in the sense defined by ICAO, which is
A generic term relating to the vertical position of an aircraft in flight and meaning variously height, altitude, or Flight Level.
Rodan has provided the reasoning behind the different clearances which may be given.