The official Shimano instructions have you push fluid up from the caliper into the cup at the lever. However, there are videos and plenty of people who apparently doing a gravity bleed. I did this myself and it seemed to work. But besides the gravity part, the broader question relates to the SRAM process having more fiddly little steps. The Shimano process is pretty much along the lines of "Drive the fluid up in to the cup, flip the lever a couple of times." Shimano spends a lot less time emphasizing chasing bubbles at the caliper and lever. I'm wondering if there is a real technological reason for this or is it just "style."