Descent below the initial altitude assigned would require a transition altitude on an approach chart or you would be required to maintain your last assigned altitude when cleared for the approach. Does anybody remember the round hill mountain crash going into Dulles 35 years ago when they descended to initial approach altitude? TWA hit round hill descending to OM crossing altitude too soon. Cleared for the approach to me means you can descend to any altitude shown on your approach plate. If you are not on an approach route you have no min altitude so stay where you were.