AFAIK, you can descend when 'established' on the localizer. I haven't seen an exact definition of established, but I think you have to at least have the localizer needle off the peg and moving toward the center. I like to have it within a dot of center, but I'm not religious about that, depending on the approach conditions. If ATC is turning me in on a short approach, very near the OM, (or the copilot has drifted more than a dot off
) I may start the descent sooner, but never without having course guidance from the localizer needle.