Most of our clearances are, "Fly HDG xxx, maintain xxxx until established, cleared ILS xx approach"
When cleared for the approach, we arm only LOC. When LOC captures we arm approach. This prevents descending on the GS prior to being on a published section of the approach.
Another point - many approaches have step down fixes prior to GS intercept. Arming approach prior to GS intercept altitude could cause non compliance with a step down fix altitude. In this case, after being cleared for the approach, we set FAF altitude and (as long as FMGC fixes/altitudes are correctly coded/confirmed) push for managed descent, after the last step down is complied with, we arm the approach.
(Another aside - when cleared for a visual, we maintain last assigned altitude until on approximately a 3 degree glidepath to the runway. Previously our pilots were descending to FAF altitude and these early descents were occasionally causing GPWS warnings.)