I always had trouble cutting my beard all the way up to my sideburns when I used a disposable. My sideburns would lengthen over time and it would bother me. It's much easier for me to do my sideburns with a DE. Here's my technique: 1)stretch the skin of my sideburn up with nonshaving hand. 2)Place the safety-bar at the very edge of my sideburn and the handle of the razor pointing straight down. 3) move the handle away from my face until the blade touches hair at a relatively obtuse angle. 4) cut a few milimeters of beard hair, resume normal angle and continue shaving.
Now my sideburns stay the same length between haircuts.