I think on at least one of those types what you do depends on the vertical mode you are using prior to the FAF.
According to one set of SOPs If you have fully checked the FMC etc, are descending in VNAV path ( not for example V/S or FLCH) on the arrival and then when cleared for the approach you can immediately set the MDA..
(I see whilst I was typing Alpine Flyer has covered the "trap")