As a former runner myself, I was always taught that it depends on speed. If you're going at a slower pace, which distance running usually falls under, then heel strike is more natural, but if you are sprinting then you want to stay on your toes. As somebody that has been a sprinter and XC runner, I've found this to be true. How that advice fares with everyone else, I can't say, but when I know that when I try running distance on my toes or heal strike while sprinting, I will not perform as well and distance on toes feels like my Achilles will snap after about a half mile or so.