DFC, when you arm the APP mode before the LOC capture is physically indicated this may lead, at least in some aircraft, to give you a handful of recovery work, nothing you want in IMC in a mountainous environment; reason is that some systems allow for descend with the G/S without being established on the LOC. On the 737 it is very simple, "cleared to establish on LOC" or "cleared for the approach" means arm the LOC, indication white.
Caption shows green asks for heading to RWY QFU.
When "cleared for approach" and when inside 1/2 deflection of LOC arm the APP, indication white. When G/S captured, caption green, reset altitude to initial G/A altitude.
Never give the automatics a chance to deviate from your altitude before you really want them to.