When you are cleared for an approach, your descent is unrestricted (unless ATC put one in). This does not mean diving toward the IAF/FAF/FAP altitude - it means this:
1. If on vectors, maintain LAST ASSIGNED ALTITUDE until established on the procedure - then descend according to that procedure. The reason you maintain the last assigned is because when under vectors, ATC provide terrain clearance to the point you can continue with a pilot interpreted aid.
2. If not on vectors, then YOU provide the terrain clearance so descend not below published minimum descent altitudes directly underneath you at any time until established on the approach - thenm as per the published profile. Obviously you might be tracking procedurally via a STAR, AREA or ERC chart etc. The MSAs are on those!