The larger the line the harder it is to push air bubbles downhill, so I assume it has to do with the location of the high spots in the system and the volume around them.  Most of the lines in the system are larger than I would think makes sense, so I tried to route the lines with as few high spots as possible.  Pushing fluid from the bottom up using just gravity seemed to work fine on my system.  The use of a vacuum bleeding system depends on all the seals working just as well to seal air out as to seal fluid in, and I suspect this is the case in our systems, but don't know for sure.  The best thing is to do whatever works, as a firm pedal is what counts.