I think it depends on the firm. My method was to mail my app materials unless the firm's website or NALP form expressly requested for applicants to 1) apply online or 2) e-mail materials.
The vast majority did not request any certain method so I mailed most of mine. I've heard that many do firms prefer e-mail because they then have an electronic copy saved immediately (without having to scan it in, etc.)But call me old-fashioned I guess...