face cuts means everything. direction of fall, speed of fall, what the stem does as it leaves the stump
a good face cut can reduce the risk of a barber chair.
the old timers knew all of this, and you can arguably be more precise with an axe then you can with a power saw, just a lot more work and skill involved.
as far as the back cut goes, accuracy isn't nearly as important as many claim it to be, ideally you want to be level or just a touch higher then the face cut, but a little low won't hurt, and a little high isn't the end of the world, it makes for an ugly butt log, and an uglier stump. That said, there are drawbacks to high and low back cuts, to low can make it barber chair easier. as well as more work to wedge it over, too high and you miss the face entirely, or over cut into your slope wood (for a standard "frowny" face cut). The benefit of an intentionally low back cut, is if your using equipment to push it over, it helps prevent shoving the butt off the stump, though if your careful and leave enough hold wood, this shouldn't be an issue.
the lesson if you've made it this far...
For most trees and most timber, a clean, well aimed face cut, where in the cuts from the aiming cut and slope cut match with out over or under lapping, as well as cleaning out all of the loose bits of wood, will make timber fall to the desired direction, much much more likely, (there are still other factors, such as natural lean, wind, limb weight etc) also when in doubt, make the face steep and deep... worry about breakage and fiber pull later
Biggest consideration for the back cut, is to get it close to level with the aiming cut, and leave hinge wood, 90% of the time keep the hold wood parallel with the aiming cut of the face, more wood on one side will pull the tree towards the fat side of the hold wood, more then you think, but not something to ever rely on