In appendix carry, I find a rearward cant more concealable and comfortable than straight vertical as there's less depth possible with the cant until it starts...interfering uncomfortably. For me a vertical cant also means in all likelihood I won't be able to bend down to sit comfortably, if at all.
A slight rearward cant also makes the butt of the firearm at a more comfortable and natural grabbing angle IMO, making a slightly faster draw (arguable certainly, but it being more natural isn't IMO as the butt is more "level" to the top of one's pants than in the straight vertical, specific holster and depth depending of course.)
But like all holsters, this is certrainly a YMMV thing, and admittedly I might feel less preference either way too if I was one of those real skinny folks too.
C-