Is it normal to start descending on the G/S before you have captured the LOC?
I'm getting confused with all the different answers here. Maybe I didn't understand the question...
It is NORMAL to fly an ILS under an IFR clearance. Given that assumption, you may NOT descend on the G/S until the LOC is captured, because you are not on an approach segment until you are on the LOC. In the US, the rule is codified in FAR 91.175(i):
"When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, shall, in addition to complying with Sec. 91.177, maintain the last altitude assigned to that pilot until the aircraft is established on a segment of a published route or instrument approach procedure unless a different altitude is assigned by ATC."
All the other circumstances noted here appear to be under other than "normal" conditions.