Angled back cuts are not good for several reasons. One is that wedges will lever the wood of the stump away from the cut and the stump wood tends to de-laminate or split more than hold. Another is that the log and stump can barberchair, or split out and roll. Another reason not to do it is that you can overcut the hinge, cut through the hinge or miss the hinge entirely. Then what do you do? It is about as bad as a Dutchman's cut in the face. Not good. Too many variables that make felling more uncontrolled.
With a straight backcut that is one to two inches above where the face cuts meet, you get tight wood perpendicular to the wedges that will hold, and you can cut to the proper depth and correct the cut, if needed. Your saw will also be perpendicular to the tree, and is easier to lever into the back cut that way (the saw will actaully want to do that naturally if you are dogged in right). You can also space more hinge on one side or the other for falling a tree more one way or another. Generally, with large diameter trees, they will pop off the hinge, and often times kick back over the stump once they are level with the ground.
I have messed with a lot of angled cuts felling a lot of smaller trees, and generally I do not advocate angled back cuts, except for 4 inch or less diameter trees. I angle the cuts on those becasue it is easy to push them over once you cut them 80% through the trunk with only the one back cut, and the front of the tree acts as the hinge. Anything bigger and I cut a notch in the face and back cut level. Sometimes I cut at an angle if the tree is leaning, and go perpendicular to the tree rather than level to the ground. Supposedly you should go level to the ground, but I dunno. I like wedges and cuts perpendicular to the tree, and not always level to the ground. The saws tend to be better at that angle as well, dogged in an me levering on the handle.