Personally, if cleared to intercept the LOC and im below the glide I fully arm the APCH mode as I take it as an "implied clearance" that will eventually follow with"descend with the ils" once I call them LOC established. worst case scenario you can always re-select NAV mode again if atc tell you not to descend with the ILS(very unlikely to happen) or VS mode the aircraft to a lower altitude if atc require you to do so unless the aircraft has fully established itself if you were to slow to press those buttons.