If the FAP and the FAF are coincident, you can start your time at the FAF and revert to a nonprecision approach if the GS subsequently fails. Personally, I don't like to do that, and would do it only if I were forced to by circumstances, but it's legal. I've had check pilots ask me why I didn't start the timer at the OM, and my reply is that it does me no good, it's just something else to distract me, because I'm not going to continue the approach if the GS fails. Losing the GS means something has failed, and I don't have time to do the troubleshooting while flying the approach. I prefer to climb, call approach, and then figure out what happened and what I have left, before starting another approach. But that's just my personal attitude, not a regulation. I haven't been failed on a checkride for this, I just think they wanted to know what I was doing and why.